We all know what to do by now
brew install libxml2 libxslt libiconv brew link libxml2 libxslt libiconv gem install nokogiri -- \ --with-xml2-include=/usr/local/Cellar/libxml2/2.7.8/include/libxml2 \ --with-xml2-lib=/usr/local/Cellar/libxml2/2.7.8/lib \ --with-xslt-dir=/usr/local/Cellar/libxslt/1.1.26 \ --with-iconv-include=/usr/local/Cellar/libiconv/1.14/include \ --with-iconv-lib=/usr/local/Cellar/libiconv/1.14/lib
If the problem still persists it's likely to be the build tools that are out of version or missing. If you replace the gcc with the gcc-4.2 the problem should go away.
sudo ln -s /usr/bin/gcc-4.2 /usr/bin/gcc