title: Working with Multiple Projects in CMake
body: <p>I am currently working on transitioning over a Visual C++ project (that has multiple subprojects inside of it) over to CMake.</p>
  
  <p>There is one thing that I am not sure about – basically, to include the subprojects from the top level CMakeLists.txt file, I just utilize the <code>add_subdirectory</code> command, and reference the directories that these various subprojects are stored under.</p>
  
  <p>However, I have one project that is in the <em>same</em> directory as my top level CMakeLists.txt file, and so I am wondering if it is still possible to include this file somehow? CMake does not allow me to call add_subdirectory on the existing PROJECT_BINARY_DIR (see below):</p>
  
  <pre><code>add_subdirectory(${PROJECT_BINARY_DIR}) #not allowed in CMake
  </code></pre>
  
  <p>I cannot think of another way for me to include this subproject into my CMake build. Any ideas?</p>