diagnostics: %empty enables -Wempty-rule

* src/complain.h, src/complain.c (warning_is_unset): New.
* src/reader.c (grammar_current_rule_empty_set): If enabled -Wempty-rule,
if not disabled.
* tests/actions.at (Implicitly empty rule): Check this feature.
Also check that -Wno-empty-rule does disable this warning.

src/complain.h

@@ -101,6 +101,9 @@ typedef enum
     Wall              = ~complaint & ~fatal & ~silent
   } warnings;
 
+/** Whether the warnings of \a flags are all unset.
+    (Never enabled, never disabled). */
+bool warning_is_unset (warnings flags);
 
 /** Make a complaint, with maybe a location.  */
 void complain (location const *loc, warnings flags, char const *message, ...)