title: Finding a directory in CMake
body: <h2>The problem:</h2>
  <p>I want to add the <a href="http://mathema.tician.de/software/boost-numeric-bindings" rel="noreferrer">boost numeric bindings</a> as an include directory in my build. This is typically compiled as:</p>
  <p><code>c++ -I/where/you/want/to/install/it/include/boost-numeric-bindings</code></p>
  <p>All of the header files I reference from my program are all relative to this directory, so in CMake I want to find this directory (wherever it is installed on the parent system) and add it to the <code>include_directories</code>.</p>
  <h2>What I'm looking for:</h2>
  <p>Something like this:</p>
  <pre><code>find_directory(BNB_INCLUDE_DIR boost-numeric-bindings)
  
  include_directories(${BNB_INCLUDE_DIR})
  </code></pre>
  <p>But the <code>find_directory</code> command does not exist. Am I missing something here?</p>
  <h2>What I've tried:</h2>
  <p>I've tried:</p>
  <pre><code>find_path(BNB_INCLUDE_DIR boost/numeric/bindings/traits/ublas_vector.hpp)
  
  include_directories(${BNB_INCLUDE_DIR})
  </code></pre>
  <p>(which is the first file I need from the library) but this gives me the full path to the file and not the path to the directory containing the entire prefix to the file specified in the command.</p>