[GtkGLExt] gtkmm 2.4 patch for gtkglextmm 1.0.1

["j." <merkaba quantumfish com>]

if it's useful to anyone, 
here's a patch against gtkglextmm 1.0.1 sources to make it work with
gtkmm 2.4.
http://sermon.quantumfish.com/files/gtkmm-2.4.patch

the examples work, but i didn't touch any of the docs.
besides the configure and makefile changes, these are the primary
differences i noticed:

- Glib::RefPtr doesn't have an is_null() method anymore
- namespace is sigc instead of SigC
- SigC::Object is deprecated. replaced with sigc::trackable
- SigC::slot is replaced with sigc::mem_fun or sigc::ptr_fun
- get_allocation() returns Gdk::Rectangle