Assorted build fixes to pass distcheck.
* clutter/glx/Makefile.am: Install the clutter-glx.h header.
* clutter/Makefile.am: Actually apply linker flags.
* configure.ac: Use a more readable CLUTTER_API_VERSION instead
of CLUTTER_MAJORMINOR; require gobject-2.0 with glib-2.0 >= 2.10,
as we are using GObject and the slice allocator from glib 2.10.
* clutter.pc.in: Add the apiversion variable.
