Configurable FFmpeg expression, fdk-aac library, license fixes

These commits do several things:

* Add ```ffmpeg_configurable```, a configurable expression for FFmpeg styled after ```vim_configurable```. Otherwise, it's difficult to configure an FFmpeg expression for users who want the command line tool in their environment.
* Add the ```fdk-aac``` library, a high-quality AAC codec implementation from Android.
* Allow ```ffmpeg_2``` and ```ffmpeg_configurable``` to use the ```fdk-aac``` library. This is disabled by default because:
* FFmpeg is GPLv2+ unless ```fdk-aac``` or ```faac``` are enabled, in which case it is non-free and unredistributable.