title: Override option in CMake subproject
body: <p>I'm trying to reuse the <code>CMakeLists.txt</code> of a third-party project whose source I don't want to change (<a href="https://github.com/alamaison/winsparkle/blob/master/3rdparty/expat/CMakeLists.txt" rel="noreferrer">expat</a>, to be exact).  I've added the project as a subproject of the top level using <code>add_subdirectory</code>.</p>
  
  <p>This works but now I would like to set the value of some of the subproject's <code>option</code>s in the top level <code>CMakeLists.txt</code>.  How do I do this?</p>