glsl: Generalize MSVC fix for strcasecmp().

glsl: Generalize MSVC fix for strcasecmp().

This will let us use strcasecmp() from anywhere inside Mesa without
having to worry about the fact that it doesn't exist in MSVC.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>