title: Shared variables in multi directory qmake
body: <p>I have multiple qmake .pro files which are called from a root .pro file with a subdirs template.</p>
  
  <p>What is the best way to change a setting for the whole build system (eg. release to debug).</p>
  
  <p>Currently I can only do this by changing each sub .pro file, or using an external script to change each .pro file.</p>
  
  <p>I was hoping there was a way to share qmake variables between the subdirs .pro file and the others.</p>