title: Error message "GWL_WNDPROC undeclared" while following this tutorial
body: <p>I am following this tutorial:
  <em><a href="http://www.winprog.org/tutorial/start.html" rel="nofollow noreferrer">theForger's Win32 API Programming Tutorial</a></em></p>
  <p>In section 4 (message looping) (<em><a href="http://www.winprog.org/tutorial/message_loop.html" rel="nofollow noreferrer">Understanding the Message Loop</a></em>), there is the code:</p>
  <pre><code>WNDPROC fWndProc = (WNDPROC)GetWindowLong(Msg.hwnd, GWL_WNDPROC);
  fWndProc(Msg.hwnd, Msg.message, Msg.wParam, Msg.lParam);
  </code></pre>
  <p>I tried to compile it, but I got this error message:</p>
  <blockquote>
  <p>GWL_WNDPROC undeclared</p>
  </blockquote>
  <p>What can I do to fix it?</p>