title: Purpose of CMake ENDIF and ELSE arguments
body: <p>In CMake the <code>ELSE</code> and <code>ENDIF</code> control flow functions take expressions as arguments. These are <a href="http://www.cmake.org/cmake/help/v3.0/command/if.html" rel="noreferrer">optional</a> according to the documentation. What is the purpose of these then? Is it just to make the original <code>IF</code> expression clearer for maintenance purposes, or does it provide some functionality?</p>