Fork ffmpeg to fix pthread issue

This commit is the last to build in the cookbook: https://github.com/FFmpeg/FFmpeg/tree/fde5c7dc79eb017790ba232442ad2a4eecea4bf1

This commit broke the build: https://github.com/FFmpeg/FFmpeg/commit/83c12fefd22fc2326a000019e5c1a33e90a874e8