Problem: unused stored value warning

Solution: add LIBZMQ_UNUSED

src/router.cpp

@@ -80,6 +80,8 @@ void zmq::router_t::xattach_pipe (pipe_t *pipe_, bool subscribe_to_all_)
 
         rc = pipe_->write (&probe_msg);
         // zmq_assert (rc) is not applicable here, since it is not a bug.
+        LIBZMQ_UNUSED (rc);
+
         pipe_->flush ();
 
         rc = probe_msg.close ();