The behaviour is a bit different from other find modules:
`FFMPEG_FIND_COMPONENTS` should contain the
list of required libraries before calling `find_package(FFmpeg)`.
If not set, a default set of most common libraries is searched for:
`avcodec avdevice avfilter avformat avutil`
http://ffmpeg.org/download.html
Ref D8855
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D8918