title: How to include an additional CMakeLists.txt
body: <p>Say we have a command call <code>foo</code> in CMakeLists.txt which is in folder <code>/A</code>.</p>
  
  <p><code>foo</code> is defined in antother CMakeLists.txt which is in folder <code>/B</code>.</p>
  
  <p>How can one reference to <code>/B/CMakeLists.txt</code> from within <code>/A/CMakeLists.txt</code> in order to call <code>foo</code>?</p>
  
  <p>I tried to set search paths to <code>/B/CMakeLists.txt</code> via:</p>
  
  <ul>
  <li><code>CMAKE_INCLUDE_PATH</code></li>
  <li><code>CMAKE_MODULE_PATH</code></li>
  <li><code>CMAKE_SOURCE_DIR</code></li>
  </ul>
  
  <p>but none of them worked.</p>
  
  <p>CMake still complaines <code>Unknown CMake command "foo".</code></p>