title: CMAKE add sub-directory which is not sub-directory on real directory
body: <p>Is It possible to include sibling directory as Sub-Directory inside cmake ?</p>
  
  <p>Something like </p>
  
  <pre><code>A 
    CMakeLists.txt
  
  B
    CMakeLists.txt
  </code></pre>
  
  <p>and B includes A as sub-directory ?</p>