glsl/glcpp: Once again report undefined macro name in error message.

glsl/glcpp: Once again report undefined macro name in error message.

While writing the previous commit message, I just felt bad documenting the
shortcoming of the change, (that undefined macro names would not be reported
in error messages).

Fix this by preserving the first-encounterd undefined macro name and reporting
that in any resulting error message.

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