title: how to install x11_xt_lib when configure VTK?
body: <p>when I configure VTK with ccmake command on Ubuntu 14.04,
  the following error message happens after I press c button.
  how to install that stuff?</p>
  
  <pre><code> CMake Error at Rendering/OpenGL/CMakeLists.txt:304 (message):
     X11_Xt_LIB could not be found.  Required for VTK X lib.
  </code></pre>