Fix pass-through of '=' and add a test for it.

Previously '=' was not included in our PUNCTUATION regeular expression,
but it *was* excldued from our OTHER regular expression, so we were
getting the default (and hamful) lex action of just printing it.

The test we add here is named "punctuator" with the idea that we can
extend it as needed for other punctuator testing.

glcpp-parse.y

@@ -426,6 +426,7 @@ operator:
 |	OR			{ $$ = OR; }
 |	';'			{ $$ = ';'; }
 |	','			{ $$ = ','; }
+|	'='			{ $$ = '='; }
 |	PASTE			{ $$ = PASTE; }
 |	DEFINED			{ $$ = DEFINED; }
 ;