[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.

changed milestone to %3.0.x maintenance

added Component::Build system: Contribs label

mentioned in merge request !7080 (merged)

added MRStatus::NotCompliant label

added MRStatus::Reviewable label and removed MRStatus::NotCompliant label

merged manually