glcpp: Reject token pasting operator in GLES

The GLSL ES 3.0 spec (Section 12.17) says:
"GLSL ES 1.00 removed token pasting and other functionality."

NOTE: This is a candidate for the stable branches.
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Carl Worth <cworth@cworth.org>

src/glsl/glcpp/glcpp-lex.l

@@ -260,6 +260,8 @@ HEXADECIMAL_INTEGER	0[xX][0-9a-fA-F]+[uU]?
 }
 
 "##" {
+	if (parser->is_gles)
+		glcpp_error(yylloc, yyextra, "Token pasting (##) is illegal in GLES");
 	return PASTE;
 }