libvorbis library version 1.3.7.
The following changes have been made:
- Obviously, unnecessary folders and files (doc, examples, m4, macos, macosx,
  symbian, test, vq) have been removed.
- https://github.com/xiph/vorbis/pull/100 and
  https://github.com/xiph/vorbis/pull/101 have been applied.
- All modifications have been marked with /* OpenMPT */.
- For building, premake is used to generate Visual Studio project files.
  See ../build/premake/ for details.
