Support for building ffmpeg with MinGW#76

WilliamVenner wants to merge 1 commit intozmwangx:masterzmwangx/rust-ffmpeg-sys:masterfrom

WilliamVenner:feat/mingw-build-ffmpegWilliamVenner/rust-ffmpeg-sys:feat/mingw-build-ffmpegCopy head branch name to clipboard