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).