title: Nokogiri Compilation Error - Can't find libraries/headers
body: <p>Trying to install a gem, but it can't find the headers, despite specifying them:</p>
  
  <pre><code>sudo gem install nokogiri -- --with-xml2-lib=/usr/local/lib --with-xml2-include=/usr/local/include/libxml2  --with-xml2-include=/usr/local/include/libxml2  --with-xslt-include=/usr/local/include/libxslt
  Building native extensions.  This could take a while...
  ERROR:  Error installing nokogiri:
      ERROR: Failed to build gem native extension.
  
  /usr/bin/ruby extconf.rb --with-xml2-lib=/usr/local/lib --with-xml2-include=/usr/local/include/libxml2 --with-xml2-include=/usr/local/include/libxml2 --with-xslt-include=/usr/local/include/libxslt
  checking for #include &lt;libxml/parser.h&gt;
  ... no
  -----
  libxml2 is missing.  please visit http://nokogiri.org/tutorials/installing_nokogiri.html for help with installing dependencies.
  </code></pre>
  
  <p>The <code>/usr/local/include/libxml2/libxml/parser.h</code> file does exist, so I don't see why this isn't working.</p>
  
  <p>EDIT: I'm on Centos 5.4</p>