title: CMake doesn't pick up INTERFACE_INCLUDE_DIRECTORIES of linked library
body: <p>I have this in my cmake-file</p>
  
  <pre><code>get_target_property(moggle_interface_includes moggle INTERFACE_INCLUDE_DIRECTORIES)
  message("Moggle interface includes: ${moggle_interface_includes}")
  
  target_link_libraries(motor
      moggle
  )
  
  get_target_property(motor_includes motor INCLUDE_DIRECTORIES)
  message("MOTOR includes ${motor_includes}")
  </code></pre>
  
  <p>Which outputs this</p>
  
  <pre><code>Moggle interface includes: "/home/nick/code/onegame/motor/moggle/include"
  Motor includes:" "
  </code></pre>
  
  <p>How can this be? When moggle is linked, it should also, according to this</p>
  
  <pre><code>CMake will also propagate "usage requirements" from linked library targets. Usage 
  requirements affect compilation of sources in the &lt;target&gt;. They are specified by 
  properties defined on linked targets. During generation of the build system, CMake 
  integrates usage requirement property values with the corresponding build properties for 
  &lt;target&gt;:
  
       INTERFACE_COMPILE_DEFINITONS: Appends to COMPILE_DEFINITONS
       INTERFACE_INCLUDE_DIRECTORIES: Appends to INCLUDE_DIRECTORIES
  </code></pre>
  
  <p>... pick up the INTERFACE_INCLUDE_DIRECTORIES and add them to motor's, so what am I doing wrong?</p>
  
  <ul>
  <li>CMake verison: cmake version 2.8.12.2</li>
  <li>OS: Arch Linux</li>
  </ul>