title: Issue between Eigen, make and cmake (probably a cmake mistake)
body: <p>I encounter an issue when trying to build a little bit of code. (I'm on linux)</p>
  
  <p>To make it simple:</p>
  
  <p>Here is what I've got in my Position.h file (at the really begining, I think the next isn't necessary to solve that issue):</p>
  
  <pre><code>#include &lt;Eigen/Dense&gt;
  </code></pre>
  
  <p>And Here is my CMakeLists.txt:</p>
  
  <pre><code>project(p)
  include_directories("./Eigen")
  add_executable(
      p
      Eigen/Dense
      Position.h # wich requires Eigen/Dense
      Position.cpp
      #other files
      )
  </code></pre>
  
  <p>In the project directory there is two directories: build and Eigen</p>
  
  <p>To create the Makefile, I go in the build directory, then, type <code>cmake ..</code>. A Makefile is created, but when I try to <code>make</code> i got the error:</p>
  
  <pre><code>/path/to/Position.h:30:23: fatal error: Eigen/Dense: no such file or directory.
  </code></pre>
  
  <p>Position.h is from a code picked up from github (I can give you the link if wanted).</p>
  
  <p>Please, can you give me a direction to search or maybe if you see what is wrong, what is my mistake</p>
  
  <p>Thanks!</p>