ffmpeg: improve cross platform support & fix licensing issues, resolves #7013

+ Removed asserts for (l)gpl & (l)gpl3 making it easier to enable and disable all
  (l)gpl & (l)gpl3 portions
+ Fix build inputs so that nonfree software isn't pulled in when building ffmpeg
+ Fix compiler on systems with clang
+ Disable libraries that fail on darwin until they can be fixed
+ Fix linux only build inputs so that they aren't pulled in on other platforms
+ Fix optional build inputs
+ Opencv causes circular dependency issues so it is disabled until it can be resolved