MKVToolNix 7.3.0

Update to [latest stable release](https://www.bunkus.org/blog/2014/10/mkvtoolnix-v7-3-0-released/), add option to build [experimental QT GUI](https://trac.bunkus.org/wiki/MkvToolNixGuiStatus), add libmagic as recommended dependency, upgrade flac dependency to recommended, downgrade gettext dependency to optional, remove unnecessary configure options locating ogg and vorbis, change format of ```drake install``` command.
@mikemcquaid Personally, I work with and create a good number of MKVs with FLAC audio (mainly because there's no open source decoder for DTS-HD MA). It seems like it would be ideal for this to support everything within the current matroska specification by default, otherwise this could choke on perfectly valid files, and users might not realize why that is.