Merge static PyGLib module into PyGI

Remove gi._glib._glib as a separately compiled module. Move all C files into
pygobject/gi. Remove compilation and use of libpyglib-gi-2.0-python.so as a
shared dependency since we do not distribute header files for it.
Remove unused threading macros.

https://bugzilla.gnome.org/show_bug.cgi?id=712197
Move Python glib options module into gi package

Move gi/_glib/option.py into gi/_option.py. Remove gi/_glib since it is no
longer needed.

https://bugzilla.gnome.org/show_bug.cgi?id=712197