Be ready for changes in bf-translations repository

Made it so if there's release/datafiles/locale/po
folder, then all the .po files will be converted
to .mo at blender compile time and installed to
an appropriate location.

Uses small own implementation msgfmt which is
based on msgfmt.py from Python project, but also
supports contexts.

There's no functional changes for until we've
switched to use source .po files instead of
pre-compiled .mo.

P.S. Well, there's one change which is msgfmt.cc
     being compiled even if it's not used, but
     would rather not clutter code with checks
     since pretty soon we'll use this program
     anyway.