scons: Fix MSYS2 Mingw-w64 build (toolchain=mingw)

This patch is based on https://github.com/msys2/MINGW-packages/blob/28e3f85e09b6947ea80036c49f6c38f1394f93ca/mingw-w64-mesa/link-ole32.patch but with tweaks to avoid MSVC build break when applied.

v2: Create Mingw platform alias pointing to windows host platform define to avoid spurious "crosscompilation";

v3: Fix obviously wrong compiler flags for swr driver. There is more work to do for swr build to be successful though.

v4: Update original patch URL because it has been relocated.

v5: Don't bother patching autools stuff as it's not used by MSYS2 Mingw-w64 build and its days are numbered anyway;

v6: After Mingw posix flag fix in 295851eb things are far simpler as we don't need more linking of uuid, ole32, version and shell32 than what is already in place.