glsl: allow GLSL compiler version to be overridden to 1.50

glsl: allow GLSL compiler version to be overridden to 1.50

Although GLSL 1.50 compiler support is not available,
this change will allow MESA_GLSL_VERSION_OVERRIDE=150 to be
used while 1.50 support is being developed.

Since no drivers claim 1.50 GLSL support, this change should
only impact Mesa when MESA_GLSL_VERSION_OVERRIDE=150 is set.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>