* acinclude.m4: only turn on FORTIFY_SOURCE if it won't anger gcc

Following libvirt commits cf9bced0 and 1c2edf0f

acinclude.m4

@@ -19,7 +19,7 @@ AC_DEFUN([AUGEAS_COMPILE_WARNINGS],[
 
     warnCFLAGS=
 
-    common_flags="-Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fasynchronous-unwind-tables"
+    common_flags="-fexceptions -fasynchronous-unwind-tables"
 
     case "$enable_compile_warnings" in
     no)
@@ -45,6 +45,14 @@ AC_DEFUN([AUGEAS_COMPILE_WARNINGS],[
 	;;
     esac
 
+    AH_VERBATIM([FORTIFY_SOURCE],
+    [/* Enable compile-time and run-time bounds-checking, and some warnings,
+        without upsetting newer glibc. */
+     #if !defined _FORTIFY_SOURCE && defined __OPTIMIZE__ && __OPTIMIZE__
+     # define _FORTIFY_SOURCE 2
+     #endif
+    ])
+
     compiler_flags=
     for option in $try_compiler_flags; do
 	SAVE_CFLAGS="$CFLAGS"