title: zGuide for zeroMQ fails compilation
body: <p>I'm trying to build the zguide c++ examples, but the compilation fails. Here is the error ouput:</p>
  
  <pre><code>asyncsrv.cpp: In member function ‘void client_task::start()’:
  asyncsrv.cpp:37:70: error: cannot convert ‘zmq::socket_t’ to ‘void*’ in initialization zmq::pollitem_t items[] = {{client_socket_, 0, ZMQ_POLLIN, 0}};
  
  asyncsrv.cpp: In member function ‘void server_task::run()’: asyncsrv.cpp:140:52: error: cannot convert ‘zmq::socket_t’ to ‘void*’ for argument ‘1’ to ‘void zmq::proxy(void*, void*, void*)’ zmq::proxy(frontend_, backend_, nullptr);
  </code></pre>