title: Running install command in CMake, but only if
body: <p>I am extremely new to CMake, and having trouble setting up an install rule.
  I want to run the following command in <code>make install</code>:</p>
  
  <pre><code>update-rc.d solshare_stats_runscript defaults
  </code></pre>
  
  <p>But I only want to run this command if:</p>
  
  <pre><code>CMAKE_INSTALL_PREFIX="/"
  </code></pre>
  
  <p>How can I do this?</p>