glsl/glcpp: Promote "extra token at end of directive" from warning to error

We've always warned about this case, but a recent confromance test expects
this to be an error that causes compilation to fail. Make it so.

Also add a "make check" test to ensure these errors are generated.

This fixes the following Khronos GLES3 conformance tests:

	invalid_conditionals.tokens_after_ifdef_vertex
	invalid_conditionals.tokens_after_ifdef_fragment
	invalid_conditionals.tokens_after_ifndef_vertex
	invalid_conditionals.tokens_after_ifndef_fragment

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/glcpp/glcpp-parse.y

@@ -625,7 +625,7 @@ replacement_list:
 junk:
 	/* empty */
 |	pp_tokens {
-		glcpp_warning(&@1, parser, "extra tokens at end of directive");
+		glcpp_error(&@1, parser, "extra tokens at end of directive");
 	}
 ;