title: Tell CLion to use header include path with prefix
body: <p>I have a project layout as follows:</p>
  
  <ul>
  <li>workspace
  
  <ul>
  <li>project_a
  
  <ul>
  <li>project_a -> .h files here</li>
  <li>Root -> .cxx files here</li>
  </ul></li>
  <li>project_b
  
  <ul>
  <li>project_b -> .h files here</li>
  <li>Root -> .cxx files here</li>
  </ul></li>
  </ul></li>
  </ul>
  
  <p>I cannot change the directory layout due to the build system that we're using.
  Headers are included as </p>
  
  <p><code>#include "project_a/some_header.h</code></p>
  
  <p>also from the corresponding .cxx file. 
  I've created a CMakeLists.txt file in the root directory, that adds all my projects via <code>include_directories(project_a project_b)</code>, which should be the path prefixed before the one given in the <code>#include</code>. CLion does not manage to find and index any of my files.</p>
  
  <p>Additionally, I have an automatically generated directory of headers of structure</p>
  
  <ul>
  <li>include
  
  <ul>
  <li>lib_a -> .h files</li>
  <li>lib_b -> .h files</li>
  </ul></li>
  </ul>
  
  <p>and I've set them up accordingly, but it also does not work.</p>
  
  <p>Does CLion not manage to resolve the prefixed path in the <code>#include</code> or why is this not working?</p>