I'm try to install google-geocode for rails

sudo gem install google-geocode

but I get the following error. Any suggestions?

Building native extensions.  This could take a while...
ERROR:  Error installing google-geocode:
    ERROR: Failed to build gem native extension.

/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/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,/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/include,/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/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,/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/include,/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/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,/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/include,/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/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,/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/include,/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/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=/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/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 /Library/Ruby/Gems/1.8/gems/nokogiri-1.4.0 for inspection.
Results logged to /Library/Ruby/Gems/1.8/gems/nokogiri-1.4.0/ext/nokogiri/gem_make.out