Don't clobber our toolchain defaults.

https://bugs.gentoo.org/898526
--- a/configure.ac
+++ b/configure.ac
@@ -141,7 +141,7 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [
 ])], have_clang=yes, have_clang=no)
 
 if test x$enable_gcc_hardening != xno; then
-    CFLAGS="$CFLAGS -D_FORTIFY_SOURCE=2"
+    CFLAGS="$CFLAGS"
     if test x$have_clang = xyes; then
        OTR_CHECK_CFLAGS(-Qunused-arguments)
     fi
@@ -166,4 +166,6 @@ if test x$enable_linker_hardening != xno; then
     OTR_CHECK_LDFLAGS(-z relro -z now, "$all_ldflags_for_check", "$all_libs_for_check")
 fi
 
+AX_ADD_FORTIFY_SOURCE
+
 AC_OUTPUT([Makefile po/Makefile.in])