Upgraded to Mountain Lion and started getting the above warning when using nokogiri.
It is happening because OSX has a default libxml2 in /usr/lib/libxml2.2.dylib
But the libxml2 headers used during nokogiri compilation are the ones in /usr/include/libxml2. nb.I don’t use macport or homebrew.
So /usr/include/libxml2 is version 2.7.3 which you can see by looking inside libxml/xmlversion.h
To get rid of the warning in nokogiri I needed to get matching headers for libxml2 2.7.8, so I downloaded ftp://xmlsoft.org/libxml2/libxml2-2.7.8.tar.gz Extracted it somewhere in my home folder (where I kept all the source files), sudo mv /usr/include/libxml2 /usr/include/libxml2-2.7.3 then symlink my extracted libxml2-2.7.8/include to /usr/include/libxml2