define a macro for heap object deletion in a unified manner (related to issue #1524)

src/zmq_utils.cpp

@@ -88,7 +88,7 @@ void zmq_threadclose(void* thread)
 {
     zmq::thread_t* pThread = static_cast<zmq::thread_t*>(thread);
     pThread->stop();
-    delete pThread;
+    LIBZMQ_DELETE(pThread);
 }
 
 //  Z85 codec, taken from 0MQ RFC project, implements RFC32 Z85 encoding