mp4: Wire it in and make it compile cleanly.

mp4: Wire it in and make it compile cleanly.

This switches to our local copy of libmp4ff. The configure script
now turns on support for the aac audio format even if libmp4ff is
not installed.

Some changes to mp4.c were necessary to silence warnings, most of
which are trivial. The single notably non-trivial change is the missing
initialization of chunk_sample and chunk in mp4ff_chunk_of_sample().