title: CMake Include third party header for a non cmake library
body: <p>I have a project set up like so:</p>
  
  <pre><code>project
    CMakeLists.txt
    src
      CMakeLists.txt
      Mylib.cpp
    thirdparty
      somelib
        include
          header.h
  </code></pre>
  
  <p>I am trying to include header.h inside my code with <code>#include "header.h"</code> however  the preprocessor cannot find the header.</p>
  
  <p>I have tried adding the third party include directory using <code>target_include_directories</code> in the src CMakeLists.txt, however that does not help. I have tried both <code>thirdparty/somelib/include</code> and <code>../thirdparty/somelib/include</code> for the path.</p>
  
  <p>How do I properly get the header on the include path?</p>