title: copy multiple files to null directory
body: <p>I want to check if source file exist, so I assign TO_DIR=/dev/null in my gnu makefile.</p>
  
  <pre><code>APPS=a b c d
  install:
    cp $(APPS) $(TO_DIR)
  </code></pre>
  
  <ul>
  <li>for normal case, I'll run 'TO_DIR=~/bin make install'</li>
  <li>for test case(just make sure file exist for copy), I'll run 'TO_DIR=/dev/null make install'</li>
  </ul>
  
  <p>But it will failed, because /dev/null is not a pseudo directory.
  Is there better solution?</p>