title: Add a dependency not in a subdirectory using CMake
body: <p>Let's say there's following directory structure:</p>
  
  <pre><code>root
    |
    +--projects
    |      |
    |      +-test
    |         |
    |         +-CMakeFiles.txt
    |
    +--libs
         |
         +-testlib
              |
              +-CMakeFiles.txt
  </code></pre>
  
  <p><code>test</code> contains <code>CMakeFiles.txt</code> and <code>testlib</code> also contains <code>CMakeFiles.txt</code>. "test" produces an executable and "testlib" produces a static library.</p>
  
  <p>I want "test" to link with "testlib" without using symlinks and without moving "testlib" library into a subdirectory within "test".</p>
  
  <p>Because "testlib" isn't a subdirectory of "test", I can't do</p>
  
  <pre><code>add_subdirectory("../../libs/testlib")
  </code></pre>
  
  <p>In test's <code>CMakeFiles.txt</code> - CMake will complain about "testlib" not being in the "test" subdirectory.</p>
  
  <p>Also, because system has several different compilers, I can't simply install "testlib" libraries into some kind of central directory, so I want <code>test</code> to compile a local copy of <code>testlib</code> and link with it (i.e. as if <code>testlib</code> was a subdirectory). I also want the "test" project to automatically rebuild "testlib" if it has been changed.</p>
  
  <p>So, how can I deal with it? I am using CMake 2.8.4 on Windows&nbsp;XP SP3.</p>