title: Why doesn't qmake add "QT += widgets" to *.pro files automatically?
body: <p>I am trying to build a simple "Hello World" example in QT5.</p>
  
  <p>I am compiling the code using the below steps</p>
  
  <pre><code>qmake-qt5 -project
  qmake-qt5
  make
  </code></pre>
  
  <p>I am getting the below error</p>
  
  <pre><code>main.cpp:1:24: fatal error: QApplication: No such file or directory
   #include &lt;QApplication&gt;
                          ^
  compilation terminated.
  </code></pre>
  
  <p>When I read QT forums, its mentioned to add "QT += widgets" to the *.pro file. After doing this the code is compiled.</p>
  
  <p><strong>Question:</strong> Why do I need to add "QT += widgets" to the *.pro file manually ? Why doesn't qmake do it automatically ?</p>
  
  <p>Note: I am using Ubuntu</p>
  
  <p><strong><em>Code</em></strong></p>
  
  <pre><code>#include &lt;QApplication&gt;
  #include &lt;QLabel&gt;
  
  int main(int argc, char *argv[])
  {
      QApplication app(argc, argv);
      QLabel *label = new QLabel("Hello Qt!");
      label-&gt;show();
      return app.exec();
  } 
  </code></pre>