title: CMake - Check if a higher level directory exists
body: <p>I'm new to CMake and I tried to do this in my root CMakeLists.txt :</p>
  
  <pre><code>set(HAVE_MY_SDK   OFF)
  
  if(IS_DIRECTORY "${PROJECT_SOURCE_DIR}../Libs/A")
      if(EXISTS "${PROJECT_SOURCE_DIR}../Libs/A/CMakeLists.txt")
          set (HAVE_MY_SDK   ON)
      endif()
  endif()
  </code></pre>
  
  <p>Currently, CMake is just avoiding the instruction, leaving <code>HAVE_MY_SDK</code> in a <code>OFF</code> status. Is it possible to check an higher level directory with CMake ? Or maybe doing it with an indirect method</p>