gnu: gnome: Add new package gtkglext

* gnu/packages/gnome.scm (gtkglext): New variable.
* gnu/packages/patches/gtkglext-disable-disable-deprecated.patch,
  gnu/packages/patches/gtkglext-remove-pangox-dependency.patch: New
  files.
* gnu-system.am (dist_patch_DATA): Add them.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>