title: CMake: include_directories not propagating from subdirectory
body: <p>I decided to include a library (more specifically <a href="http://code.google.com/p/yaml-cpp/" rel="nofollow noreferrer" title="yaml-cpp">yaml-cpp</a>) in my project. Since both the  projects (yaml-cpp and mine) are using CMAKE for building I decided to try and place the yaml-cpp directory inside my root directory and just use <code>add_subdirectory(yaml-cpp)</code> and <code>target_link_libraries(${EXECUTABLE_NAME} foolib yaml-cpp)</code>, since yaml-cpp's CMakeLists.txt already contains</p>
  <pre><code>include_directories(${YAML_CPP_SOURCE_DIR}/include)
  
  # ...
  
  add_library(yaml-cpp
      ${sources}
      ${public_headers}
      ${private_headers}
      ${contrib_sources}
      ${contrib_public_headers}
      ${contrib_private_headers}
  )
  </code></pre>
  <p>...and I should be fine on both includes and the library target. But as it turns out, upon compilation I get the following error:</p>
  <blockquote>
  <p>P:\zpp\TheGameShow\TGS.cpp(16): fatal error C1083: Cannot open include file: 'yaml-cpp/yaml.h': No such file or directory</p>
  </blockquote>
  <p>...unless I duplicate the line <code>include_directories(${YAML_CPP_SOURCE_DIR}/include)</code> in my own CMakeLists.txt</p>
  <p>My question is: Is it the desired behavior? Shouldn't the include directories propagate from subdirectories to root directories? Am I doing something wrong?</p>
  <p><strong>edit:</strong> I am using Visual Studio 2010 and CMake 2.8.6</p>