Add support for the new libmpcdec API which magically gets us support for SV8 files...

Add support for the new libmpcdec API which magically gets us support for SV8 files. Also do some random cleanup. Fix...

Original commit message from CVS:
* configure.ac:
* ext/musepack/gstmusepackdec.c: (gst_musepackdec_base_init),
(gst_musepackdec_init), (gst_musepackdec_dispose),
(gst_musepackdec_handle_seek_event), (gst_musepack_stream_init),
(gst_musepackdec_loop), (plugin_init):
* ext/musepack/gstmusepackdec.h:
* ext/musepack/gstmusepackreader.c:
* ext/musepack/gstmusepackreader.h:
Add support for the new libmpcdec API which magically gets us support
for SV8 files. Also do some random cleanup. Fixes bug #526905.