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>

src/glsl/glcpp/glcpp.h

@@ -39,7 +39,7 @@
 
 typedef struct expression_value {
 	intmax_t value;
-	bool has_undefined;
+	char *undefined_macro;
 } expression_value_t;