glsl: Treat layout-qualifier-id's as case-insensitive in desktop GLSL.

In desktop GLSL, location qualifiers are case-insensitive.  In GLSL
ES, they are case-sensitive.  This patch handles the difference by
using a new function to match layout qualifiers,
match_layout_qualifier(), which calls either strcmp() or strcasecmp()
as appropriate.

Fixes piglit tests:
- layout-not-case-sensitive-in.geom
- layout-not-case-sensitive-max-vert.geom
- layout-not-case-sensitive-out.geom
- layout-not-case-sensitive.frag