gnu: mesa: Move out of xorg.scm and into gl.scm

* gnu/packages/xorg.scm (mesa): Move to...
* gnu/packages/gl.scm (mesa): ... here.  New variable.
* gnu/packages/gnome.scm, gnu/packages/gnuzilla.scm,
  gnu/packages/qemu.scm, gnu/packages/qt.scm: Adjust accordingly.

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