FFmpeg with libfdk_aac

As stated [here](https://trac.ffmpeg.org/wiki/Encode/AAC): _“the Fraunhofer FDK AAC codec library is currently the highest-quality AAC encoder available with ffmpeg.”_ It should be better than the native FFmpeg AAC encoder.

I don’t know if compiling FFmpeg with libfdk_aac imply some license violations, otherwise can I compile it myself on my iPad? If yes, where I can find some clues?