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

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 02354690ee02dfa05564137aafc6721eb54d21ab)