title: Setting QT_DEBUG_PLUGINS fails
body: <p>I have read this blogpost <a href="http://www.ics.com/blog/qt-tips-and-tricks-part-1" rel="nofollow">http://www.ics.com/blog/qt-tips-and-tricks-part-1</a> and tried to enable plugin debugging as described.</p>
  
  <p>I've put this line in my main.cpp:</p>
  
  <p><s>
      qputenv(QT_DEBUG_PLUGINS, 1);</p>
  
  <p>But if I try to compile I'm getting this error:</p>
  
  <pre><code>.../src/main.cpp:14: error: 'QT_DEBUG_PLUGINS' was not declared in this scope
  qputenv(QT_DEBUG_PLUGINS, -1);
  </code></pre>
  
  <p>What is the problem here and how do I have to do it right?
  </s></p>
  
  <pre><code>qputenv("QT_DEBUG_PLUGINS", QByteArray("1"));
  </code></pre>
  
  <p>But I don't get any additional output.</p>
  
  <p>I'm using Qt5.5.1 with QtCreator 3.6 under KUbuntu 15.10.</p>