mcfgthreads: enable cross-compiling on Darwin

using -Werror is problematic in general, remove for all platforms
fixes #97214

pkgs/os-specific/windows/mcfgthreads/default.nix

@@ -18,6 +18,10 @@ stdenv.mkDerivation {
     rm -r "$sourceRoot/debug" "$sourceRoot/release"
   '';
 
+  postPatch = ''
+    substituteInPlace Makefile.am --replace '-Werror' ''
+  '';
+
   nativeBuildInputs = [
     autoreconfHook
   ];