fix build error with libzmq-3.2.5 (#103)

error zmq_send_const undefined and zmq_msg_gets undefined

Author: sqbing

zmq.hpp

@@ -372,13 +372,15 @@ namespace zmq
             return a == b;
         }
 
+#if ZMQ_VERSION >= ZMQ_MAKE_VERSION(4, 0, 8)
         inline const char* gets(const char *property_)
         {
             const char* value = zmq_msg_gets (&msg, property_);
             if (value == NULL)
                 throw error_t ();
             return value;
         }
+#endif
 
     private:
         //  The underlying message
@@ -622,7 +624,11 @@ namespace zmq
 
         inline size_t send (const void *buf_, size_t len_, int flags_ = 0)
         {
+#if ZMQ_VERSION >= ZMQ_MAKE_VERSION(4, 0, 8)
             int nbytes = zmq_send_const (ptr, buf_, len_, flags_);
+#else
+            int nbytes = zmq_send (ptr, buf_, len_, flags_);
+#endif
             if (nbytes >= 0)
                 return (size_t) nbytes;
             if (zmq_errno () == EAGAIN)