title: `cfg` which is always true / false?
body: <p>For testing purpose I need <code>cfg</code> which is always true / false.
  For true I use</p>
  <pre class="lang-rust prettyprint-override"><code>#[ cfg( target_pointer_width = &quot;64&quot;) ]
  ...
  </code></pre>
  <p>But obviously it is not general enough.
  What is optimal way of expressing <code>cfg</code> to get necessary value?</p>