Fix mcfgthread Windows cross-build: add windres wrapper

mcfgthread 2.1.1 in nixpkgs-2511 switched from autotools to meson.
meson's import('windows').compile_resources() needs windres to compile
version.rc, but the package only creates a dlltool wrapper. The prefixed
x86_64-w64-mingw32-windres exists in the cross-bintools but meson
searches for the unprefixed name. Add a windres wrapper using the same
pattern as the existing dlltool wrapper.

Fixes all 34 remaining Windows build failures in eval 424.

Author: angerman

flake.nix

@@ -116,6 +116,21 @@
                (prev.lib.cmakeBool "ENABLE_LIB_ONLY" true)
              ];
            });
+
+           # mcfgthread 2.1.1 (nixpkgs-2511) switched from autotools to meson.
+           # meson's import('windows').compile_resources() requires `windres`
+           # but the package only creates a dlltool wrapper, not windres.
+           # The prefixed x86_64-w64-mingw32-windres exists in the cross-bintools
+           # but meson searches for the unprefixed name. Same pattern as dlltool.
+           windows = prev.windows // {
+             mcfgthreads = prev.windows.mcfgthreads.overrideAttrs (old: {
+               nativeBuildInputs = old.nativeBuildInputs ++ [
+                 (prev.writeScriptBin "windres" ''
+                   ${prev.stdenv.cc.targetPrefix}windres "$@"
+                 '')
+               ];
+             });
+           };
          });
 
          musl = (final: prev: prev.lib.optionalAttrs prev.stdenv.hostPlatform.isMusl {