title: Is there a possibility to pass includes via -isystem when using qmake
body: <p>I use qmake as a makefile builder and want to stick to it.
  Further I would like to use "gcc -Wall -Werror -Wundef -Wextra" to get robust code.
  I'm thinking about "-pedantic" but that's further up the road.
  My main problem at the moment are the tons of warnings generated by libraries like 
  boost, parts of qt and the like.</p>
  
  <p>At the moment I use pragmas whenever I include warning-generating headers   </p>
  
  <pre><code>    #pragma GCC diagnostic ignored "-Wall"
      #include &lt;QtGui&gt;
      ...
      #include &lt;QWidget&gt;
      #pragma GCC diagnostic error "-Wall"
  </code></pre>
  
  <p>This is far from cute, rather tedious and cumbersome especially as other programmers have to do so too.
  Is there an option using qmake that allows to include qt-libraries as system headers, 
  thus supressing their warnings. For plain makefiles and cmake I knwow -isystem but I cannot find a qmake pendant for this.</p>