[3.0] contrib: build postproc as part of ffmpeg
If GPL is enabled, build postproc together with ffmpeg instead of building an ffmpeg-less ancient version with numerous bugs.
Should fix crashes on macOS and possibly other OSes when postproc is used.
(cherry picked from commit 599191d3) (edited)
- 3.0 didn't have the ranlib patch
The old git keeps failing in the CI, and we don't even need it. This patch was done when 4.0 was using the same FFmpeg 4.4.