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).