title: CPLUS_INCLUDE_PATH doesn't works
body: <p>I have set my environment variable <code>CPLUS_INCLUDE_PATH</code> to <code>boost</code>.</p>
  
  <pre><code>&gt;echo $CPLUS_INCLUDE_PATH
  boost
  </code></pre>
  
  <p>But it doesn't work at all.</p>
  
  <pre><code>&gt;g++ parse_utils.cpp
  parse_utils.cpp:1:34: fatal error: boost/lexical_cast.hpp: No such file or directory
  </code></pre>
  
  <p>The <code>.hpp</code> is in that path.</p>
  
  <pre><code>&gt;g++ parse_utils.cpp -Iboost 
  (work well)
  </code></pre>
  
  <p>I thought <code>CPLUS_INCLUDE_PATH</code> is equivalent to <code>-I</code> option?</p>
  
  <pre><code>&gt;g++ --version
  g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
  </code></pre>