title: Can't include an external library in another header with Cmake
body: <p>I have a project setup with the following folder structure:</p>
  
  <ul>
  <li>External
  <ul>
  <li>Boost</li>
  <li>Poco</li>
  </ul></li>
  <li>Libraries
  <ul>
  <li>Lib1
  <ul>
  <li>src
  <ul>
  <li>lib1.cpp</li>
  </ul></li>
  <li>CMakeLists.txt</li>
  </ul></li>
  <li>Lib2
  <ul>
  <li>src</li>
  <li>CMakeLists.txt</li>
  </ul></li>
  </ul></li>
  <li>Projects
  <ul>
  <li>Project1
  <ul>
  <li>src</li>
  <li>CMakeLists.txt</li>
  </ul></li>
  <li>Project
  <ul>
  <li>src</li>
  <li>CMakeLists.txt</li>
  </ul></li>
  </ul></li>
  <li>Public
  <ul>
  <li>Lib1
  <ul>
  <li>lib1.h</li>
  </ul></li>
  <li>Lib2</li>
  </ul></li>
  </ul>
  
  <p>Basically I have a folder for external libraries like Poco or Boost. Next to that I have a projects folder. Each project has it own independent source files and have the possibility to include/link one or more of the libs in the Libraries folder. Each library has also a private src folder and a public header folder.  </p>
  
  <p>The CMakeLists.txt file of a library looks something like this (${PUBLIC_DIRECTORY} is set to the correct public folder)</p>
  
  <pre><code>set(Sources
      "${PUBLIC_DIRECTORY}/lib1.h"
      "src/lib1.cpp"        
  )
  
  include_directories("../../External/Boost")
  add_library(Lib1 ${Sources})
  link_directories("${CMAKE_CURRENT_BINARY_DIR}/Lib1")
  </code></pre>
  
  <p>The problem happens when I try to include a boost header  like boost/lexical_cast.hpp.
  When I include this in lib1.cpp, everything compiles. But when I do the same in lib1.h, I get the error: boost/lexical_cast.hpp: No such file or directory.</p>
  
  <p>Does anyone know how to fix this problem? </p>
  
  <p>thanks in advance</p>