ffmpeg: allow customization of codecs, (de)muxers and other components

Add the option to customize the list of decoders, encoders, muxers, demuxers,
parsers, protocols, bsfs and filters to be built into ffmpeg, and to compile or
exclude input and output devices.

[Peter: fix qstrip invocation]
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>