title: When is cmakes add_custom_command POST_BUILD being triggered?
body: <p>Lets consider that I have registered a custom command for a target:</p>
  
  <pre><code>add_custom_command(TARGET ${target_name} POST_BUILD
                     COMMAND cmake -E copy_if_different ${file_path} ${dest_file}
                     COMMENT "exporting ${file_path} to ${dest_file}"
                     VERBATIM)
  </code></pre>
  
  <p>What I am struggling to find out is when the command is actually being run.
  Always after the target has been checked like this ? </p>
  
  <pre><code>[ 10%] Built target libzmq
  </code></pre>
  
  <p>Even if <em>'nothing has to be done'</em> ?</p>
  
  <p>Or just in case that for the target something had to be done ?</p>