mingw: Disalbe FORTIFY_SOURCE by default.

In newer versions of mingw, programs compiled with FORTIFY_SOURCE need
to link to libssp or they will have link-time errors.

gmp has been broken since @pstn updated mingw-64 in c60a0b0447

pkgs/build-support/cc-wrapper/default.nix

@@ -472,7 +472,7 @@ stdenv.mkDerivation {
     + optionalString hostPlatform.isCygwin ''
       hardening_unsupported_flags+=" pic"
     '' + optionalString targetPlatform.isMinGW ''
-      hardening_unsupported_flags+=" stackprotector"
+      hardening_unsupported_flags+=" stackprotector fortify"
     '' + optionalString targetPlatform.isAvr ''
       hardening_unsupported_flags+=" stackprotector pic"
     '' + optionalString (targetPlatform.libc == "newlib") ''