Add RCS Id.
Does not really use libmpcdec. Remove it. Bump PKGREVISION.
Simplify. Nothing of this logic is needed to successfully build gxmms2,
the only user of this file.
Switch to musepack.
While here, make this thing package.
I don't think all options were or are now correctly depicted in the PLIST,
but I'll leave that to some other commit.