warnings: used but undeclared symbols are warnings

We used to raise an error if a symbol appears only in a %printer or
%destructor.  Make it a warning.

* src/symtab.h (status): New enum.
(symbol): Replace the binary "declared" with the three-state "status".
Adjust dependencies.
* src/symtab.c (symbol_check_defined): Needed symbols are an error,
whereas "used" are simply warnings.
* src/symlist.c (symbol_list_destructor_set, symbol_list_printer): Set
symbol status to 'used' when associated to destructors or printers.
* input.at (Undeclared symbols used for a printer or destructor): New.

src/symtab.h

@@ -51,6 +51,13 @@ typedef int symbol_number;
 
 typedef struct symbol symbol;
 
+typedef enum
+  {
+    needed,   /**< found but not "defined".  */
+    used,     /**< used by %printer but not declared.  */
+    declared, /**< defined with %type or %token.  */
+  } status;
+
 /* When extending this structure, be sure to complete
    symbol_check_alias_consistency.  */
 struct symbol
@@ -90,7 +97,7 @@ struct symbol
      symbol-string pair for an alias.  */
   symbol *alias;
   symbol_class class;
-  bool declared;
+  status status;
 };
 
 /** Undefined user number.  */