build ffmpeg from a stable version

[dunn]

What's the reason for building the HEAD of the ffmpeg repository instead of the latest stable version?

[acozine]

Partly following online documentation, partly wanting to use the latest of everything, and partly not wanting to have zillions of version variables.
The approach has lasted this long because we have not seen failures with ffmpeg itself, only with the libraries it uses.
That said, I'm in favor of specifying a version.

[dunn]

31f220e