intel_extensions: Add ability to set GLSL version via environment

intel_extensions: Add ability to set GLSL version via environment

Add ability to set the GLSL version used by the GLcontext by setting the
environment variable INTEL_GLSL_VERSION. For example,
    env INTEL_GLSL_VERSION=130 prog args
If the environment variable is missing, the GLSL versions defaults to 120.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>