Merge 'mingw-w64' and 'darwin' into cross-win-osx.

Both branches have quite a lot in common, so it's time for a merge and
do the cleanups with respect to both implementations and also generalize
both implementations as much as possible.

This also closes #1876.

Conflicts:
	pkgs/development/interpreters/lua-5/5.2.nix
	pkgs/development/libraries/SDL/default.nix
	pkgs/development/libraries/glew/default.nix
	pkgs/top-level/all-packages.nix

pkgs/development/tools/build-managers/cmake/default.nix

@@ -26,7 +26,13 @@ stdenv.mkDerivation rec {
   patches =
     # Don't search in non-Nix locations such as /usr, but do search in
     # Nixpkgs' Glibc. 
-    optional (stdenv ? glibc) ./search-path.patch;
+    optional (stdenv ? glibc) ./search-path.patch ++
+    optional (stdenv ? cross) (fetchurl {
+      name = "fix-darwin-cross-compile.patch";
+      url = "http://public.kitware.com/Bug/file_download.php?"
+          + "file_id=4981&type=bug";
+      sha256 = "16acmdr27adma7gs9rs0dxdiqppm15vl3vv3agy7y8s94wyh4ybv";
+    });
 
   buildInputs = [ curl expat zlib bzip2 libarchive ]
     ++ optional useNcurses ncurses