isisd: change ISIS_METHOD to use C preprocessor

this fixes warnings from vtysh extract.pl by making sure the isis method
files always 'work'. (previously, extract.pl would grab unselected isis
method sources and then complain about missing headers)
diff --git a/configure.ac b/configure.ac
index 1a5051a..e7c99b0 100755
--- a/configure.ac
+++ b/configure.ac
@@ -782,15 +782,18 @@
 dnl --------------------------
 dnl Determine IS-IS I/O method
 dnl --------------------------
+AC_DEFINE(ISIS_METHOD_PFPACKET,	1, [ constant value for isis method pfpacket ])
+AC_DEFINE(ISIS_METHOD_DLPI,	2, [ constant value for isis method dlpi ])
+AC_DEFINE(ISIS_METHOD_BPF,	3, [ constant value for isis method bpf ])
 AC_CHECK_HEADER(net/bpf.h)
 AC_CHECK_HEADER(sys/dlpi.h)
 AC_MSG_CHECKING(zebra IS-IS I/O method)
 if test x"$opsys" = x"gnu-linux"; then
   AC_MSG_RESULT(pfpacket)
-  ISIS_METHOD=isis_pfpacket.o
+  ISIS_METHOD_MACRO="ISIS_METHOD_PFPACKET"
 elif test x"$opsys" = x"sol2-6" -o x"$opsys" = x"sol8"; then
   AC_MSG_RESULT(DLPI)
-  ISIS_METHOD="isis_dlpi.o"
+  ISIS_METHOD_MACRO="ISIS_METHOD_DLPI"
 else
   if test $ac_cv_header_net_bpf_h = no; then
     if test $ac_cv_header_sys_dlpi_h = no; then
@@ -800,13 +803,13 @@
     else
       AC_MSG_RESULT(DLPI)
     fi
-    ISIS_METHOD="isis_dlpi.o"
+    ISIS_METHOD_MACRO="ISIS_METHOD_DLPI"
   else
     AC_MSG_RESULT(BPF)
-    ISIS_METHOD="isis_bpf.o"
+    ISIS_METHOD_MACRO="ISIS_METHOD_BPF"
   fi
 fi
-AC_SUBST(ISIS_METHOD)
+AC_DEFINE_UNQUOTED(ISIS_METHOD, $ISIS_METHOD_MACRO, [ selected method for isis, == one of the constants ])
 
 dnl ------------------------------------
 dnl check for broken CMSG_FIRSTHDR macro