title: How to avoid rebuilding existing wheels when using pip?
body: <p>With <code>pip 1.5.X</code>, we can use <code>pip wheel</code> to build and cache a wheel of a package, then use <code>--use-wheel</code> with <code>pip install</code> to install from the cached wheel.</p>
  
  <p>I'm trying to use this feature in a environment setup script.
  This is what I'm trying:</p>
  
  <pre><code>pip wheel --wheel-dir=/tmp Cython==0.19.2
  pip install Cython==0.19.2 --use-wheel --no-index --find-links=/tmp
  </code></pre>
  
  <p>I'm expecting <code>pip wheel</code> to check if the wheel already exists before building it. But it seems to ignore the existing wheel and build every single time.</p>
  
  <p>Is it possible to avoid this?</p>