scons: Fix MSYS2 Mingw-w64 build

Reviewed-by: Jose Fonseca <>

This patch is based on 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;

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 it's 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.
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merge request