title: CMake external project fixed revision repeated download
body: <p>I'm using CMake's External Project support very similar as proposed in
  <a href="https://stackoverflow.com/questions/9689183/cmake-googletest">CMake + GoogleTest</a>.</p>
  
  <p>Obviously my build eventually depends on a download step and hence will only work if that is possible - i.e. I need an internet connection. However, if I am using a fixed revison to download (not HEAD) and have already build the external project once, I'm not sure why this download step seems to be performed every time.</p>
  
  <p>While no actual files seem to be downloaded and no time consuming rebuild is triggered, my build still fails when I'm not online. This is pretty annoying when working without internet connection, e.g. on a train.</p>
  
  <p>Is there a way to let my build skip the download step altogether if it can be deduced to be unnessary (fixed revision + lib for the EP already build / no clean step performed)?</p>