build FFmpeg with CMake, update version

[smanders]

currently we build FFmpeg on Linux via configure/make and have a pre-built version on Windows

• linux: https://github.com/smanders/externpro/blob/18.01.1/projects/ffmpeg_2.6.2.cmake
• windows: https://github.com/smanders/externpro/blob/18.01.1/projects/ffmpeg_2.6.2.1.cmake
• https://github.com/smanders/externpro/blob/18.01.1/projects/ffmpeg.cmake#L60-L63

to enable us to move forward to new versions of FFmpeg we should build FFmpeg with CMake (so it can build the same way on all platforms without requiring mingw or some other unix-like layer on Windows)

and once we're building via CMake, update to a newer version of FFmpeg

[smanders]

initial work now in dev branch (will be part of externpro 23.03 release)

• cmake build, work-in-progress d63c88b
• smanders FORK 4.3.1 15597c1
• don't build 4.3.1 by default 099eefc

FFmpeg/FFmpeg@n4.3.1...ndrasmussen:FFmpeg:xp4.3.1
FFmpeg/FFmpeg@n4.3.1...smanders:FFmpeg:xp4.3.1