glcpp: Add a test for #elif with an undefined macro.

glcpp: Add a test for #elif with an undefined macro.

As written, this test correctly raises an error for #elif being used
with an undefined macro (and not as an argument to "defined"). If the
preceding #if were '#if 1' then this diagnositc would correctly be
hidden. That allows code such as the following to not raise an error:

	#ifndef MAYBE_UNDEFINED
	#elif MAYBE_UNDEFINED < 5
	...
	#endif

So this test case is working as expected already. We add it here just
to improve test coverage.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Carl Worth <cworth@cworth.org>