Taken Debian's patch and removed docs matches:
    https://salsa.debian.org/toolchain-team/gcc.git
Also see https://bugs.gentoo.org/621036 where
initially Gentoo used too complicated macro.

# DP: Turn on -D_FORTIFY_SOURCE=2 by default for C, C++, ObjC, ObjC++,
# DP: if the optimization level is > 0
--- a/gcc/c-family/c-cppbuiltin.c
+++ b/gcc/c-family/c-cppbuiltin.c
@@ -951,6 +951,12 @@ c_cpp_builtins (cpp_reader *pfile)
   builtin_define_with_value ("__REGISTER_PREFIX__", REGISTER_PREFIX, 0);
   builtin_define_with_value ("__USER_LABEL_PREFIX__", user_label_prefix, 0);
 
+#if !defined(ACCEL_COMPILER)
+  /* Fortify Source enabled by default for optimization levels > 0 */
+  if (optimize)
+    builtin_define_with_int_value ("_FORTIFY_SOURCE", 2);
+#endif
+
   /* Misc.  */
   if (flag_gnu89_inline)
     cpp_define (pfile, "__GNUC_GNU_INLINE__");