title: Compilation error using Eigen library
body: <p>I'm trying to use the <a href="http://eigen.tuxfamily.org/dox/GettingStarted.html" rel="nofollow">Eigen</a> C++ template library for linear algebra, I compiled and installed the library to the following path <code>/usr/local/include</code></p>
  
  <p>I took a sample code from their site and I tried to compile with and without the <code>-l</code> option.</p>
  
  <pre><code>g++ -l /usr/local/include/eigen3 test.cpp -o mytest
  </code></pre>
  
  <p>but I'm getting the following error:</p>
  
  <pre><code>test.cpp:2:23: fatal error: Eigen/Dense: No such file or directory
  </code></pre>
  
  <p>How can I include this library?</p>