title: Where does one set ExternalProject properties?
body: <p>In the <code>ExternalProject_Add</code> documentation for cmake, it mentions setting the <code>EP_BASE</code> property. I've attempted setting this using the <code>set</code> keyword, however it doesn't seem to work.</p>
  
  <p>How should this property be set for <code>ExternalProject_Add</code> to use it? </p>