glsl: Move error message inside validation check reducing duplicate message handling

v2 (Paul Berry <stereotype441@gmail.com): Fix precedence error in call
to _mesa_glsl_error().

Reviewed-by: Paul Berry <stereotype441@gmail.com>
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.