title: How do I have premake run an external tool only if the target is out of date
body: <p>As part of my build process, I have an archive that I need to extract.  I can use <code>prebuildcommands</code> to always extract it, but that is repetitive and will slow the build.  </p>
  
  <p>How do say "run this command only if the extracted directory does not exist".  </p>
  
  <p>It would be good to also extract the file if the archive is new than the directory, but I'm less concerned about that, as I do not expect the archive to change often.</p>
  
  <p>Edit: I have an external requirement to use Premake 4.</p>