title: can't build matplotlib (png package issue)
body: <p>try to build matplotlib on fedora-18, build fails with</p>
  
  <pre><code>...
   png: no  [pkg-config information for 'libpng' could not
                          be found.]
  ...
                          * The following required packages can not be built:
                          * png
  </code></pre>
  
  <p>What should I do/check to build png package ?</p>