cross-gcc: disable `langCC' when targeting MinGW

pkgs/top-level/all-packages.nix

@@ -2072,6 +2072,11 @@ let
         if crossSystem != null && crossSystem.config == "i586-pc-gnu"
         then gnu.libpthreadCross
         else null;
+
+      # XXX: We have troubles cross-compiling libstdc++ on MinGW (see
+      # <http://hydra.nixos.org/build/4268232>), so don't even try.
+      langCC = (crossSystem == null
+                || crossSystem.config != "i686-pc-mingw32");
      });
     libc = libcCross;
     binutils = binutilsCross;