improve aac encoder build options and change some aac encoder defaults

By default, faac is no longer built.
Libav aac is now the default aac encoder on all platforms except osx
where ca_aac is the default.
fdk-aac is now built by default (except on osx).


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5581 b64f7644-9d1e-0410-96f1-a4d463321fa5