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+# ===========================================================================
+# http://www.gnu.org/software/autoconf-archive/ax_sys_weak_alias.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_SYS_WEAK_ALIAS
+#
+# DESCRIPTION
+#
+#   Determines whether weak aliases are supported on the system, and if so,
+#   what scheme is used to declare them. Also checks to see if aliases can
+#   cross object file boundaries, as some systems don't permit them to.
+#
+#   Most systems permit something called a "weak alias" or "weak symbol."
+#   These aliases permit a library to provide a stub form of a routine
+#   defined in another library, thus allowing the first library to operate
+#   even if the other library is not linked. This macro will check for
+#   support of weak aliases, figure out what schemes are available, and
+#   determine some characteristics of the weak alias support -- primarily,
+#   whether a weak alias declared in one object file may be referenced from
+#   another object file.
+#
+#   There are four known schemes of declaring weak symbols; each scheme is
+#   checked in turn, and the first one found is prefered. Note that only one
+#   of the mentioned preprocessor macros will be defined!
+#
+#   1. Function attributes
+#
+#   This scheme was first introduced by the GNU C compiler, and attaches
+#   attributes to particular functions. It is among the easiest to use, and
+#   so is the first one checked. If this scheme is detected, the
+#   preprocessor macro HAVE_SYS_WEAK_ALIAS_ATTRIBUTE will be defined to 1.
+#   This scheme is used as in the following code fragment:
+#
+#     void __weakf(int c)
+#     {
+#       /* Function definition... */
+#     }
+#
+#     void weakf(int c) __attribute__((weak, alias("__weakf")));
+#
+#   2. #pragma weak
+#
+#   This scheme is in use by many compilers other than the GNU C compiler.
+#   It is also particularly easy to use, and fairly portable -- well, as
+#   portable as these things get. If this scheme is detected first, the
+#   preprocessor macro HAVE_SYS_WEAK_ALIAS_PRAGMA will be defined to 1. This
+#   scheme is used as in the following code fragment:
+#
+#     extern void weakf(int c);
+#     #pragma weak weakf = __weakf
+#     void __weakf(int c)
+#     {
+#       /* Function definition... */
+#     }
+#
+#   3. #pragma _HP_SECONDARY_DEF
+#
+#   This scheme appears to be in use by the HP compiler. As it is rather
+#   specialized, this is one of the last schemes checked. If it is the first
+#   one detected, the preprocessor macro HAVE_SYS_WEAK_ALIAS_HPSECONDARY
+#   will be defined to 1. This scheme is used as in the following code
+#   fragment:
+#
+#     extern void weakf(int c);
+#     #pragma _HP_SECONDARY_DEF __weakf weakf
+#     void __weakf(int c)
+#     {
+#       /* Function definition... */
+#     }
+#
+#   4. #pragma _CRI duplicate
+#
+#   This scheme appears to be in use by the Cray compiler. As it is rather
+#   specialized, it too is one of the last schemes checked. If it is the
+#   first one detected, the preprocessor macro
+#   HAVE_SYS_WEAK_ALIAS_CRIDUPLICATE will be defined to 1. This scheme is
+#   used as in the following code fragment:
+#
+#     extern void weakf(int c);
+#     #pragma _CRI duplicate weakf as __weakf
+#     void __weakf(int c)
+#     {
+#       /* Function definition... */
+#     }
+#
+#   In addition to the preprocessor macros listed above, if any scheme is
+#   found, the preprocessor macro HAVE_SYS_WEAK_ALIAS will also be defined
+#   to 1.
+#
+#   Once a weak aliasing scheme has been found, a check will be performed to
+#   see if weak aliases are honored across object file boundaries. If they
+#   are, the HAVE_SYS_WEAK_ALIAS_CROSSFILE preprocessor macro is defined to
+#   1.
+#
+#   This Autoconf macro also makes two substitutions. The first, WEAK_ALIAS,
+#   contains the name of the scheme found (one of "attribute", "pragma",
+#   "hpsecondary", or "criduplicate"), or "no" if no weak aliasing scheme
+#   was found. The second, WEAK_ALIAS_CROSSFILE, is set to "yes" or "no"
+#   depending on whether or not weak aliases may cross object file
+#   boundaries.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Kevin L. Mitchell <klmitch@mit.edu>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 6
+
+AU_ALIAS([KLM_SYS_WEAK_ALIAS], [AX_SYS_WEAK_ALIAS])
+AC_DEFUN([AX_SYS_WEAK_ALIAS], [
+  # starting point: no aliasing scheme yet...
+  ax_sys_weak_alias=no
+
+  # Figure out what kind of aliasing may be supported...
+  _AX_SYS_WEAK_ALIAS_ATTRIBUTE
+  _AX_SYS_WEAK_ALIAS_PRAGMA
+  _AX_SYS_WEAK_ALIAS_HPSECONDARY
+  _AX_SYS_WEAK_ALIAS_CRIDUPLICATE
+
+  # Do we actually support aliasing?
+  AC_CACHE_CHECK([how to create weak aliases with $CC],
+                 [ax_cv_sys_weak_alias],
+ [ax_cv_sys_weak_alias=$ax_sys_weak_alias])
+
+  # OK, set a #define
+  AS_IF([test $ax_cv_sys_weak_alias != no], [
+    AC_DEFINE([HAVE_SYS_WEAK_ALIAS], 1,
+              [Define this if your system can create weak aliases])
+  ])
+
+  # Can aliases cross object file boundaries?
+  _AX_SYS_WEAK_ALIAS_CROSSFILE
+
+  # OK, remember the results
+  AC_SUBST([WEAK_ALIAS], [$ax_cv_sys_weak_alias])
+  AC_SUBST([WEAK_ALIAS_CROSSFILE], [$ax_cv_sys_weak_alias_crossfile])
+])
+
+AC_DEFUN([_AX_SYS_WEAK_ALIAS_ATTRIBUTE],
+[ # Test whether compiler accepts __attribute__ form of weak aliasing
+  AC_CACHE_CHECK([whether $CC accepts function __attribute__((weak,alias()))],
+  [ax_cv_sys_weak_alias_attribute], [
+    # We add -Werror if it's gcc to force an error exit if the weak attribute
+    # isn't understood
+    AS_IF([test $GCC = yes], [
+      save_CFLAGS=$CFLAGS
+      CFLAGS=-Werror])
+
+    # Try linking with a weak alias...
+    AC_LINK_IFELSE([
+      AC_LANG_PROGRAM([
+void __weakf(int c) {}
+void weakf(int c) __attribute__((weak, alias("__weakf")));],
+        [weakf(0)])],
+      [ax_cv_sys_weak_alias_attribute=yes],
+      [ax_cv_sys_weak_alias_attribute=no])
+
+    # Restore original CFLAGS
+    AS_IF([test $GCC = yes], [
+      CFLAGS=$save_CFLAGS])
+  ])
+
+  # What was the result of the test?
+  AS_IF([test $ax_sys_weak_alias = no &&
+         test $ax_cv_sys_weak_alias_attribute = yes], [
+    ax_sys_weak_alias=attribute
+    AC_DEFINE([HAVE_SYS_WEAK_ALIAS_ATTRIBUTE], 1,
+              [Define this if weak aliases may be created with __attribute__])
+  ])
+])
+
+AC_DEFUN([_AX_SYS_WEAK_ALIAS_PRAGMA],
+[ # Test whether compiler accepts #pragma form of weak aliasing
+  AC_CACHE_CHECK([whether $CC supports @%:@pragma weak],
+  [ax_cv_sys_weak_alias_pragma], [
+
+    # Try linking with a weak alias...
+    AC_LINK_IFELSE([
+      AC_LANG_PROGRAM([
+extern void weakf(int c);
+@%:@pragma weak weakf = __weakf
+void __weakf(int c) {}],
+        [weakf(0)])],
+      [ax_cv_sys_weak_alias_pragma=yes],
+      [ax_cv_sys_weak_alias_pragma=no])
+  ])
+
+  # What was the result of the test?
+  AS_IF([test $ax_sys_weak_alias = no &&
+         test $ax_cv_sys_weak_alias_pragma = yes], [
+    ax_sys_weak_alias=pragma
+    AC_DEFINE([HAVE_SYS_WEAK_ALIAS_PRAGMA], 1,
+              [Define this if weak aliases may be created with @%:@pragma weak])
+  ])
+])
+
+AC_DEFUN([_AX_SYS_WEAK_ALIAS_HPSECONDARY],
+[ # Test whether compiler accepts _HP_SECONDARY_DEF pragma from HP...
+  AC_CACHE_CHECK([whether $CC supports @%:@pragma _HP_SECONDARY_DEF],
+  [ax_cv_sys_weak_alias_hpsecondary], [
+
+    # Try linking with a weak alias...
+    AC_LINK_IFELSE([
+      AC_LANG_PROGRAM([
+extern void weakf(int c);
+@%:@pragma _HP_SECONDARY_DEF __weakf weakf
+void __weakf(int c) {}],
+        [weakf(0)])],
+      [ax_cv_sys_weak_alias_hpsecondary=yes],
+      [ax_cv_sys_weak_alias_hpsecondary=no])
+  ])
+
+  # What was the result of the test?
+  AS_IF([test $ax_sys_weak_alias = no &&
+         test $ax_cv_sys_weak_alias_hpsecondary = yes], [
+    ax_sys_weak_alias=hpsecondary
+    AC_DEFINE([HAVE_SYS_WEAK_ALIAS_HPSECONDARY], 1,
+              [Define this if weak aliases may be created with @%:@pragma _HP_SECONDARY_DEF])
+  ])
+])
+
+AC_DEFUN([_AX_SYS_WEAK_ALIAS_CRIDUPLICATE],
+[ # Test whether compiler accepts "_CRI duplicate" pragma from Cray
+  AC_CACHE_CHECK([whether $CC supports @%:@pragma _CRI duplicate],
+  [ax_cv_sys_weak_alias_criduplicate], [
+
+    # Try linking with a weak alias...
+    AC_LINK_IFELSE([
+      AC_LANG_PROGRAM([
+extern void weakf(int c);
+@%:@pragma _CRI duplicate weakf as __weakf
+void __weakf(int c) {}],
+        [weakf(0)])],
+      [ax_cv_sys_weak_alias_criduplicate=yes],
+      [ax_cv_sys_weak_alias_criduplicate=no])
+  ])
+
+  # What was the result of the test?
+  AS_IF([test $ax_sys_weak_alias = no &&
+         test $ax_cv_sys_weak_alias_criduplicate = yes], [
+    ax_sys_weak_alias=criduplicate
+    AC_DEFINE([HAVE_SYS_WEAK_ALIAS_CRIDUPLICATE], 1,
+              [Define this if weak aliases may be created with @%:@pragma _CRI duplicate])
+  ])
+])
+
+dnl Note: This macro is modeled closely on AC_LINK_IFELSE, and in fact
+dnl depends on some implementation details of that macro, particularly
+dnl its use of _AC_MSG_LOG_CONFTEST to log the failed test program and
+dnl its use of ac_link for running the linker.
+AC_DEFUN([_AX_SYS_WEAK_ALIAS_CROSSFILE],
+[ # Check to see if weak aliases can cross object file boundaries
+  AC_CACHE_CHECK([whether $CC supports weak aliases across object file boundaries],
+  [ax_cv_sys_weak_alias_crossfile], [
+    AS_IF([test $ax_cv_sys_weak_alias = no],
+          [ax_cv_sys_weak_alias_crossfile=no], [
+dnl Must build our own test files...
+      # conftest1 contains our weak alias definition...
+      cat >conftest1.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+      cat confdefs.h >>conftest1.$ac_ext
+      cat >>conftest1.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+
+@%:@ifndef HAVE_SYS_WEAK_ALIAS_ATTRIBUTE
+extern void weakf(int c);
+@%:@endif
+@%:@if defined(HAVE_SYS_WEAK_ALIAS_PRAGMA)
+@%:@pragma weak weakf = __weakf
+@%:@elif defined(HAVE_SYS_WEAK_ALIAS_HPSECONDARY)
+@%:@pragma _HP_SECONDARY_DEF __weakf weakf
+@%:@elif defined(HAVE_SYS_WEAK_ALIAS_CRIDUPLICATE)
+@%:@pragma _CRI duplicate weakf as __weakf
+@%:@endif
+void __weakf(int c) {}
+@%:@ifdef HAVE_SYS_WEAK_ALIAS_ATTRIBUTE
+void weakf(int c) __attribute((weak, alias("__weakf")));
+@%:@endif
+_ACEOF
+      # And conftest2 contains our main routine that calls it
+      cat >conftest2.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+      cat confdefs.h >> conftest2.$ac_ext
+      cat >>conftest2.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+
+extern void weakf(int c);
+int
+main ()
+{
+  weakf(0);
+  return 0;
+}
+_ACEOF
+      # We must remove the object files (if any) ourselves...
+      rm -f conftest2.$ac_objext conftest$ac_exeext
+
+      # Change ac_link to compile *2* files together
+      save_aclink=$ac_link
+      ac_link=`echo "$ac_link" | \
+               sed -e 's/conftest\(\.\$ac_ext\)/conftest1\1 conftest2\1/'`
+dnl Substitute our own routine for logging the conftest
+m4_pushdef([_AC_MSG_LOG_CONFTEST],
+[echo "$as_me: failed program was:" >&AS_MESSAGE_LOG_FD
+echo ">>> conftest1.$ac_ext" >&AS_MESSAGE_LOG_FD
+sed "s/^/| /" conftest1.$ac_ext >&AS_MESSAGE_LOG_FD
+echo ">>> conftest2.$ac_ext" >&AS_MESSAGE_LOG_FD
+sed "s/^/| /" conftest2.$ac_ext >&AS_MESSAGE_LOG_FD
+])dnl
+      # Since we created the files ourselves, don't use SOURCE argument
+      AC_LINK_IFELSE(, [ax_cv_sys_weak_alias_crossfile=yes],
+                     [ax_cv_sys_weak_alias_crossfile=no])
+dnl Restore _AC_MSG_LOG_CONFTEST
+m4_popdef([_AC_MSG_LOG_CONFTEST])dnl
+      # Restore ac_link
+      ac_link=$save_aclink
+
+      # We must remove the object files (if any) and C files ourselves...
+      rm -f conftest1.$ac_ext conftest2.$ac_ext \
+            conftest1.$ac_objext conftest2.$ac_objext
+    ])
+  ])
+
+  # What were the results of the test?
+  AS_IF([test $ax_cv_sys_weak_alias_crossfile = yes], [
+    AC_DEFINE([HAVE_SYS_WEAK_ALIAS_CROSSFILE], 1,
+              [Define this if weak aliases in other files are honored])
+  ])
+])