This is necessary for plugins when building libxul for GTK+ 3
because libxul will link against GTK+ 3 and some plugins link
against GTK+ 2, but both GTK+ libraries can't be loaded in the
same process. With this change, we have an indirection between
libxul and libgtk, named libmozgtk. plugin-container will
be modified to load libmozgtk2 in order to only have GTK+ 2
in its address space, thus enabling various plugins (e.g. flash)
on GTK+ 3 firefox.
