title: CMake: C++ include relative to base directory
body: <p>Say I have the following file structure:</p>
  
  <pre><code>project/
    - src/
      - main.cpp
      moduleA/
        - moduleA.cpp
        - moduleA.hpp
      moduleB/
        - moduleB.cpp
        - moduleB.hpp
  </code></pre>
  
  <p>Within moduleB I would like to use moduleA. As such, I would like to include <code>moduleA.hpp</code> but as a relative path to <code>src</code>. Aka, I would like to be able to write <code>#include moduleA/moduleA.hpp</code> to use moduleA. I have control over all files, and can put <code>CMakeLists.txt</code> or <code>FindModule&lt;&gt;.cmake</code> in any directories. </p>
  
  <p>Note: I want to do this because there are files in each module that have the same name (in this case, <code>parameters.hpp</code>). Therefore, I would like to be able to include <code>parameters.hpp</code> relative to <code>src</code> directory for readability purposes, and to ensure that the correct parameters file is being included. </p>
  
  <p>If this is confusing, or any other information is required, please ask. Thanks for all the help everyone!</p>