title: Setting flycheck-clang-include-path in .dir-locals using projectile
body: <p>Trying to set flycheck-clang-include-path without the need to include the full path of the project include directories using projectile, but I get errors... So this works:</p>
  
  <pre><code>((nil . (
       (company-clang-arguments . (
                       "/home/user/Downloads/project/headers"
                       "/home/user/Downloads/project/source/mon"
                       ))
           (flycheck-clang-include-path . (
                       "/home/user/Downloads/project/headers"
                       "/home/user/Downloads/project/source/mon"
                       ))
       )))
  </code></pre>
  
  <p>But this does not:</p>
  
  <pre><code>((nil . (
       (company-clang-arguments . (
                       (concat "-I" (projectile-project-root) "headers")
                       (concat "-I" (projectile-project-root) "source/mon")
                       ))
           (flycheck-clang-include-path . (
                       (concat (projectile-project-root) "headers")
                       (concat (projectile-project-root) "source/mon")
                       ))
       )))
  </code></pre>
  
  <p>The error that is reported:</p>
  
  <pre><code>flycheck-substitute-argument: Value ((concat (projectile-project-root) "headers")) of flycheck-clang-include-path for option "-I" is not a list of strings
  </code></pre>