title: Install of Nokogiri fails though libxml2 was installed
body: <p><code>gem install nokogiri</code> fails with <code>libxml2 is missing</code>.</p>
  
  <p>Details of the message show:</p>
  
  <pre><code>checking for xmlParseDoc() in libxml/parser.h... no
  checking for xmlParseDoc() in -lxml2... no
  checking for xmlParseDoc() in -llibxml2... no
  </code></pre>
  
  <p>This is in spite of the fact that I've confirmed the installation of libxml2.</p>
  
  <p>All efforts at passing the library's location using the command line flag have failed.</p>
  
  <p>I'm running:</p>
  
  <ul>
  <li>Ruby 2.2.0p0 (2014-12-25 revision 49005) [x86_64-darwin14]</li>
  <li>Mac Yosemite</li>
  <li>Installed with homebrew</li>
  </ul>