Jabberwocky

If, like me, you get the following error message:

Install the library or try one of the following options to extconf.rb:
--with-xml2-dir=/path/to/libxml2
--with-xml2-lib=/path/to/libxml2/lib
--with-xml2-include=/path/to/libxml2/include

The solution is very, very simple.
sudo apt-get install libxml2
sudo apt-get install libxml2-dev

(and maybe an sudo ldconfig -v for good measure)
(especially that last one gets the C includes needed to build libxml-ruby).