ffmpeg: let ffmpeg choose necessary parsers for audio-dec and custom

ffmpeg: let ffmpeg choose necessary parsers for audio-dec and custom

This change allows ffmpeg's configure script to determine the necessary parsers based on
selected decoders & encoders. For libffmpeg-audio-dec, this changes from our selecting:

aac, ac3, flac, mpegaudio, opus and aac_latm

to ffmpeg's configure script selecting:

aac_latm, ac3, flac, mpegaudio

for minor space savings and a simpler Makefile to manage on our end.

In libffmpeg-custom, don't select any parsers when choosing decoders/encoders. I'm undecided
on removing the parser selection in its entirety here.

Signed-off-by: Ian Leonard <antonlacon@gmail.com>