suggest: support ZMQ_SNDZMQ in zmq >= 3

GitHub: refs #63

See also: http://www.zeromq.org/docs:3-1-upgrade

Reported by Naoya Inada. Thanks!!!

    groonga_suggest_learner.c: In function ‘send_to_httpd’:
    groonga_suggest_learner.c:379:41: error: ‘ZMQ_HWM’ undeclared (first use in this function)
               zmq_setsockopt(zmq_send_sock, ZMQ_HWM, &hwm, sizeof(uint64_t));
                                             ^
    groonga_suggest_learner.c:379:41: note: each undeclared identifier is reported only once for each function it appears in