Improve AAC codec detection

[julien-celya]

The old test fails for some version of the ffmpeg binary :

• ffmpeg -version |grep enable-libfdk-aac will not work for the debian ffmpeg binary
• ffmpeg -decoders |grep 'A....D aac' will find if ffmpeg is able to decode AAC with both binaries.

No check if ffmpeg can encode AAC. If it's necessary, it's easy to add it : ffmpeg -encoders |grep 'A..... aac'

Adding -loglevel quiet makes ffmpeg generates less (useless in this case) logs.