Don't clobber our toolchain defaults.

https://bugs.gentoo.org/895680
--- a/configure.ac
+++ b/configure.ac
@@ -151,7 +151,7 @@ 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
@@ -181,6 +181,8 @@ 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 src/Makefile toolkit/Makefile tests/Makefile tests/utils/Makefile libotr.pc
            tests/utils/tap/Makefile
            tests/unit/Makefile