testzebra: pragma weak: detect systems with weak alias and provide alternative

LLVM clang does not support #pragma weak (bug 3679) on OS X. There are
other systems where the #pragma weak has varying syntax.

Added m4 file from the autoconf archives:
http://www.gnu.org/software/autoconf-archive/ax_sys_weak_alias.html

Fix up zebra/*_null.c files to use #pragma weak alias or stub functions
if not available. It's incomplete in that the different format #pragma
enable easier fixes on need.

Tested on 64bit OS X 10.7, FreeBSD 9.0 amd64 & i386 (32bit) using
gcc & clang. Tested on linux 64bit.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/zebra/misc_null.c b/zebra/misc_null.c
index c8cc47d..0680726 100644
--- a/zebra/misc_null.c
+++ b/zebra/misc_null.c
@@ -7,9 +7,15 @@
 #include "zebra/zebra_fpm.h"
 
 void ifstat_update_proc (void) { return; }
+#ifdef HAVE_SYS_WEAK_ALIAS_PRAGMA
 #pragma weak rtadv_config_write = ifstat_update_proc
 #pragma weak irdp_config_write = ifstat_update_proc
 #pragma weak ifstat_update_sysctl = ifstat_update_proc
+#else
+void rtadv_config_write (struct vty *vty, struct interface *ifp) { return; }
+void irdp_config_write (struct vty *vty, struct interface *ifp) { return; }
+void ifstat_update_sysctl (void) { return; }
+#endif
 
 void
 zfpm_trigger_update (struct route_node *rn, const char *reason)