sudo gem install therubyracer
Building native extensions.  This could take a while...
ERROR:  Error installing therubyracer:
        ERROR: Failed to build gem native extension.

        /usr/bin/ruby1.9.1 extconf.rb
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
        from extconf.rb:1:in `<main>'


Gem files will remain installed in /var/lib/gems/1.9.1/gems/therubyracer-0.12.0 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/therubyracer-0.12.0/ext/v8/gem_make.out
larsemil@gimli:~$ ruby --version
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-linux]