gcc: Build MinGW stage two with threading library

Currently this is set up to be mcfgthreads, but it could be something
else instead.

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

@@ -3,16 +3,21 @@
 stdenv.mkDerivation {
   pname = "mcfgthreads";
   version = "git";
+
   src = fetchFromGitHub {
     owner = "lhmouse";
     repo = "mcfgthread";
     rev = "9570e5ca7b98002d707c502c919d951bf256b9c6";
     sha256 = "10y2x3x601a7c1hkd6zlr3xpfsnlr05xl28v23clf619756a5755";
   };
+
+  outputs = [ "out" "dev" ];
+
   # Don't want prebuilt binaries sneaking in.
   postUnpack = ''
     rm -r "$sourceRoot/debug" "$sourceRoot/release"
   '';
+
   nativeBuildInputs = [
     autoreconfHook
   ];