daniel@Daniel-Server:~$ sudo gem install mysql2
Fetching: mysql2-0.3.18.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing mysql2:
        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:2:in `<main>'


Gem files will remain installed in /var/lib/gems/1.9.1/gems/mysql2-0.3.18 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/mysql2-0.3.18/ext/mysql2/gem_make.out
daniel@Daniel-Server:~$