#489 ac3 encoder fails to load on some installation

[Jehan]

libav is quite of a mess because of the various implementations (ffmpeg libav, libav fork, etc.) and in particular there are now 2 ac3 codecs, one with input and one float. Also as a consequence of this, the old integer implementation may be renamed as ac3_fixed. So "ac3" may be either the old or the new implementation depending on your fork/version.

So depending on what fork or version of libav/ffmpeg is installed on your computer, the current implementation might or might not manage to load the AC3 encoder.
The attached patch should make dvdstyler work with AC3, whatever the implementation and whatever the version, in any environment.

[Jehan]

Fixing AC3 encoder support

[NtAlex]

Thanks for the patch!