Add (lib)fdk-aac support?

[diizzyy]

Unless I'm reading the code incorrectly it doesn't seem to support this library at all via ffmpeg, as it is superior it would be nice if it would be an option and/or possible default one.

[wiiaboo]

If you disable the built-in aac encoder and only include fdk-aac when compiling ffmpeg, I think -o aac will use it, but not sure.

[diizzyy]

I guess that works but it's a bit inconvenient.

[cyanreg]

Closing this. I am rewriting ffmpeg's aac encoder anyway.