buildenv/osx: add xz to the build; needed to extract flac.
buildenv/osx: update BerkeleyDB to 5.3.28.
buildenv/osx: update Boost to 1.55.0.
buildenv/osx: update protobuf to 2.5.0.
buildenv/osx: add zeroc-patch.mcpp.2.7.2 patch to libmcpp build.
buildenv/osx: update libvorbis to 1.3.4.
buildenv/osx: update FLAC to 1.2.1.
buildenv/osx: update libogg to 1.3.1.
buildenv/osx: update libsndfile's .pc file to use Requires.private.

...Instead of Libs.private.

This worked when Ogg, Vorbis and FLAC would spell out their dependencies
explicitly in Libs (or its .private equivalent) as well, but that's not
the case any more. They now use Requires.private.