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

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>