Add package wavpack

WavPack is an open audio compression format that offers lossless,
lossy, and hybrid compression modes. (Hybrid mode: A lossily
compressed file is accompanied by a correction file that allows
lossless reconstruction of the original audio.) The library has a
three-clause BSD licence. Website: http://www.wavpack.com/

Built via autotools. No special dependencies; libiconv is only used
for the command-line programs, not for the library.
Build xine-lib with wavpack support

Activate wavpack support in xine-lib. This reveals a latent bug
(missing include) in xine-lib, for which a patch is provided.