Don't clobber our toolchain defaults.

https://bugs.gentoo.org/898240
--- a/configure.ac
+++ b/configure.ac
@@ -96,7 +96,9 @@ CLOCK_DEFINES='-DDATA_DIR="\"$(datadir)\"" -DPKGDATA_DIR="\"$(pkgdatadir)\""'
 AC_SUBST([CLOCK_DEFINES])
 
 dnl CFLAGS
-CFLAGS="$CFLAGS -Wall -pedantic -std=c99 -fno-strict-aliasing -fmessage-length=0 -D_FORTIFY_SOURCE=2"
+CFLAGS="$CFLAGS -Wall -pedantic -std=c99 -fno-strict-aliasing -fmessage-length=0"
+
+AX_ADD_FORTIFY_SOURCE
 
 AC_CONFIG_FILES([
 	Makefile
