buildenv/1.3.x/win32-static: add patch to silence unused parameter warnings in protobuf headers.
buildenv/1.3.x/win32-static: fix libflac.build - flac 1.3.1 now includes official .vcxprojs, so avoid regenerating them ouresvles.

Without this, our static build failed to link proprely
due to missing AVX2 symbols. The AVX2-specific file
wasn't built by FLAC's .vcproj, but it was built by
the new, official, .vcxproj file.