Revert 2009-12-30 change for undefined %prec token complaints.

That is, keep them as warnings because that should be sufficient
to satisfy POSIX without creating backward compatibility issues.
Suggested by Richard Stallman at
<http://lists.gnu.org/archive/html/bison-patches/2010-03/msg00033.html>.
* NEWS (2.5): Remove mention of complaint.
* src/reader.c (grammar_rule_check): Convert warning back to
complaint.
* tests/input.at (%prec's token must be defined): Update.
(cherry picked from commit 02354690ee)

src/reader.c

@@ -327,8 +327,8 @@ grammar_rule_check (const symbol_list *r)
   if (r->ruleprec
       && r->ruleprec->tag[0] != '\'' && r->ruleprec->tag[0] != '"'
       && !r->ruleprec->declared && !r->ruleprec->prec)
-    complain_at (r->location, _("token for %%prec is not defined: %s"),
-                 r->ruleprec->tag);
+    warn_at (r->location, _("token for %%prec is not defined: %s"),
+             r->ruleprec->tag);
 }