glcpp: Make undefined macros illegal in #if and #elif for GLES3

Simply emitting a nicely-formatted error message if any undefined macro is
encountered in a parser context expecting an expression.

With this commit, the following piglit test now passes:

	spec/glsl-es-3.00/compiler/undefined-macro.vert

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/glcpp/glcpp-parse.y

@@ -363,6 +363,8 @@ integer_constant:
 expression:
 	integer_constant
 |	IDENTIFIER {
+		if (parser->is_gles)
+			glcpp_error(& @1, parser, "undefined macro %s in expression (illegal in GLES)", $1);
 		$$ = 0;
 	}
 |	expression OR expression {