title: Cmake cannot find environment variable
body: <p>I'm building OpenHEVC (<a href="https://github.com/OpenHEVC/openHEVC" rel="nofollow">https://github.com/OpenHEVC/openHEVC</a>) with cmake on windows 7 to a VS2010 project.</p>
  
  <p>In the CMakeLists.txt at the root directory, I find the line: </p>
  
  <pre><code>include_directories(. ${SDL_INCLUDE_DIR} wrapper_hm/src/Lib)
  </code></pre>
  
  <p>The "${SDL_INCLUDE_DIR}" I think should be the SDL library(<a href="http://www.libsdl.org/download-1.2.php" rel="nofollow">http://www.libsdl.org/download-1.2.php</a>). </p>
  
  <p>My problem is, after downloading SDL library and setting up a new environment variable called "SDL_INCLUDE_DIR" Containing SDL's path (I can "echo %SDL_INCLUDE_DIR%"  in a commandline).  CMake just keeps giving me this (even after restart Cmake):</p>
  
  <p>'''
  CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
  Please set them or make sure they are set and tested correctly in the CMake files:
  SDL_INCLUDE_DIR
     used as include directory in directory C:/Users/Administrator/Desktop/Pure265/openHEVC-master/openHEVC-master
     used as include directory in directory C:/Users/Administrator/Desktop/Pure265/openHEVC-master/openHEVC-master/wrapper_hm</p>
  
  <p>Configuring incomplete, errors occurred!
  '''</p>
  
  <p>I'm confused why cmake can't find the environment variable. 
  (
  I'm new to Cmake, But I checked it here : <a href="http://www.cmake.org/Wiki/CMake_Useful_Variables" rel="nofollow">http://www.cmake.org/Wiki/CMake_Useful_Variables</a>
  That ${SDL_INCLUDE_DIR} should be reflected to the env variables.
  )</p>