Log Message:
Find native X libGL.so.2 as well as modular X libGL.so.1.

While here, update to cogl to 1.20.0

Cogl 1.20.0                                                        2015-03-23

  List of changes since Cogl 1.18.2

  Support for "quad buffer" based stereo rendering
  Support for Mir window system
  A number of updates for the Wayland support
  Improved vblank sync when using glXWaitForMSC

Many thanks to:

  Marco Trevisan (Trevino)
  Owen W. Taylor
  Robert Bragg
  Rui Matos
  Adel Gadllah
  Chris Wilson
  Necdet Yuel
  Sebastian Rasmussen
  Seong-ho Cho
  Ting-Wei Lan