title: Delete directory before compilation
body: <p>I am trying to delete a directory containing sub-directories and files before the compilation starts from a qmake pro file:</p>
  <pre><code>if exists($$OUT_PWD/android-build/src) del_tree($$OUT_PWD/android-build/src)
  </code></pre>
  <p>I tried different commands like <code>QMAKE_DEL_TREE</code> and <code>$(DEL_TREE)</code> but nothing worked. How can I delete an eventually existing directory?</p>