title: CCmake available variables
body: <p>I want to set configuration variable (path, in particular) in CMakeList.txt for my project, which user can override in CCmake.<br>
  But if i just use 
  <code>set(VAR &lt;value&gt;)</code>
  <code>VAR</code> is not available for changing in CCMAKE, how can i do that?</p>