title: qmake: handling options for both gcc and msvc
body: <p>I would like to make a project file that would work both with msvc and gcc.</p>
  
  <p>For instances, to optimize for speed you would do <code>cl /O2</code> and <code>g++ -O3</code>.</p>
  
  <p>But I don't know how to tell the project file to make the difference. I would like something like : </p>
  
  <pre><code>msvc:QMAKE_CXXFLAGS_RELEASE += /O2 /openmp /arch:AVX
  else:QMAKE_CXXFLAGS_RELEASE += -O3 -march=native -fopenmp -D_GLIBCXX_PARALLEL
  </code></pre>
  
  <p>which doesn't work sadly.</p>
  
  <p>Other way would be to change the mkspecs and bundle the new ones with my app, but it is not very portable.
  Thanks !</p>