libxml2: Added --HEAD and fixed Python bindings

-  Python 2.7+ is needed for current version. A fix to
   build with Python 2.6 has been recently added to
   the master:
   https://git.gnome.org/browse/libxml2/commit/?id=722923bc9b788f028e4221c77138fc66735a3113

-  So for system python on 10.6, you'll have to use
   the --HEAD for now.