Don't clobber toolchain defaults.

https://bugs.gentoo.org/892992
--- a/configure.ac
+++ b/configure.ac
@@ -109,7 +109,8 @@ if test "${GCC}" = yes; then
     AX_APPEND_LINK_FLAGS([-Wl,-z,now])
     AX_APPEND_LINK_FLAGS([-Wl,-z,noexecstack])
 fi
-AX_APPEND_COMPILE_FLAGS([-D_FORTIFY_SOURCE=2])
+
+AX_ADD_FORTIFY_SOURCE
 
 AC_MSG_NOTICE([**************************************** libtool])
 LT_INIT([disable-static])