mesa: Allow override of GL version with environment variable

mesa: Allow override of GL version with environment variable

It is necessary to manually set the GL version to 3.0 in order to run
Piglit tests that use glGetUniform*().

This patch allows one to override the version of the OpenGL context by
setting the environment variable MESA_GL_VERSION_OVERRIDE.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Chad Versace <chad@chad-versace.us>