title: Use environment variable to set `include_directories`
body: <p>I have an environment variable that contains paths to manually 'installed' header only libraries:</p>
  
  <pre><code>export INCLUDE_PATH="/some/path":"${INCLUDE_PATH}"
  </code></pre>
  
  <p>I want to use this in my <code>CMakeLists.txt</code>. But when I do:</p>
  
  <pre><code>include_directories("$ENV{INCLUDE_PATH}")
  </code></pre>
  
  <p>the paths appear not be properly added (no CMake error, but the compiler does not know where to look).</p>