title: Cmake Custom target that runs only once
body: <p>I need to somehow generate <code>.fidl</code> files based on <code>.cpp</code> and <code>.hpp</code> files.
  The problem is that if I try to use <code>add_custom_target</code> it runs every time.
  For <code>add_custom_command</code> I need specify output files, but I do not want to do it.
  I'd like to do something like this:</p>
  
  <pre><code>add_custom_command(gen_fidl
                     DEPENDS "*.fidl"
                     COMMAND &lt;My Commands&gt;)
  </code></pre>
  
  <p>But in this case I need specify a rule for <code>.fidl</code> files</p>
  
  <p>How would I be able to do something like this?</p>