title: Errors installing some gems on Snow Leopard - libxml2
body: <p>After upgrading to Snow Leopard, I found that installing some gems no longer worked, including webrat, mechanize etc.  Each time I get this error:</p>
  
  <pre><code>checking for xmlParseDoc() in -lxml2... no
  libxml2 is missing.  try 'port install libxml2' or 'yum install libxml2'
  *** extconf.rb failed ***
  Could not create Makefile due to some reason, probably lack of
  necessary libraries and/or headers.  Check the mkmf.log file for more
  details.  You may need configuration options.
  </code></pre>
  
  <p>And no, I haven't tried installing it with macports, because I forswore macports a couple years back when I discovered I couldn't easily downgrade/manage ports.  Since then, I have become very comfortable building from source by hand, and this is the first time I've run into something beyond my ken.  I am willing to be persuaded to go back to macports, but only if there's no easy solution to this I am missing.</p>
  
  <p>It doesn't appear that libxml2 is actually missing, just out of date perhaps?...</p>
  
  <p><strong>extended output</strong></p>
  
  <pre><code>ERROR: Failed to build gem native extension.
  
  /usr/local/bin/ruby extconf.rb
  checking for iconv.h in /opt/local/include/,/opt/local/include/libxml2,/opt/local/include,/opt/local/include,/opt/local/include/libxml2,/usr/local/include,/usr/local/include/libxml2,/usr/local/include,/usr/local/include/libxml2,/usr/include,/usr/include/libxml2... yes
  checking for libxml/parser.h in /opt/local/include/,/opt/local/include/libxml2,/opt/local/include,/opt/local/include,/opt/local/include/libxml2,/usr/local/include,/usr/local/include/libxml2,/usr/local/include,/usr/local/include/libxml2,/usr/include,/usr/include/libxml2... yes
  checking for libxslt/xslt.h in /opt/local/include/,/opt/local/include/libxml2,/opt/local/include,/opt/local/include,/opt/local/include/libxml2,/usr/local/include,/usr/local/include/libxml2,/usr/local/include,/usr/local/include/libxml2,/usr/include,/usr/include/libxml2... yes
  checking for libexslt/exslt.h in /opt/local/include/,/opt/local/include/libxml2,/opt/local/include,/opt/local/include,/opt/local/include/libxml2,/usr/local/include,/usr/local/include/libxml2,/usr/local/include,/usr/local/include/libxml2,/usr/include,/usr/include/libxml2... yes
  checking for xmlParseDoc() in -lxml2... no
  libxml2 is missing.  try 'port install libxml2' or 'yum install libxml2'
  *** extconf.rb failed ***
  Could not create Makefile due to some reason, probably lack of
  necessary libraries and/or headers.  Check the mkmf.log file for more
  details.  You may need configuration options.
  
  Provided configuration options:
      --with-opt-dir
      --without-opt-dir
      --with-opt-include
      --without-opt-include=${opt-dir}/include
      --with-opt-lib
      --without-opt-lib=${opt-dir}/lib
      --with-make-prog
      --without-make-prog
      --srcdir=.
      --curdir
      --ruby=/usr/local/bin/ruby
      --with-iconv-dir
      --without-iconv-dir
      --with-iconv-include
      --without-iconv-include=${iconv-dir}/include
      --with-iconv-lib
      --without-iconv-lib=${iconv-dir}/lib
      --with-xml2-dir
      --without-xml2-dir
      --with-xml2-include
      --without-xml2-include=${xml2-dir}/include
      --with-xml2-lib
      --without-xml2-lib=${xml2-dir}/lib
      --with-xslt-dir
      --without-xslt-dir
      --with-xslt-include
      --without-xslt-include=${xslt-dir}/include
      --with-xslt-lib
      --without-xslt-lib=${xslt-dir}/lib
      --with-xml2lib
      --without-xml2lib
  
  
  Gem files will remain installed in /usr/local/lib/ruby/gems/1.8/gems/nokogiri-1.4.1 for inspection.
  Results logged to /usr/local/lib/ruby/gems/1.8/gems/nokogiri-1.4.1/ext/nokogiri/gem_make.out
  </code></pre>