- Git clone from ffmpeg's repo.
- Checkout the revision that matches the Windows build.

./configure --arch=i386 --cc='clang -m32' --cxx='clang++ -m32' --prefix=/Build/ffmpeg-mac --disable-vda --enable-decoder=mjpeg
make
make install

Then copy the .a files from /Build/ffmpeg-mac/lib to the ffmpeg directory.