I recently upgraded a laptop of mine from Intrepid Ibex to the Jaunty Jackalope beta, and wanted to get Rails development up to speed on there. However it seems that I fell into the usual pit hole when installing Ruby on Ubuntu that simply going...
sudo apt-get install ruby
...isnt enough, and doing so will produce this lovely error...
extconf.rb:8:in `require’: no such file to load — mkmf (LoadError)
So, to combat this, you need to do instead...
sudo apt-get install ruby1.8-dev
or for Ruby 1.9...
sudo apt-get install ruby1.9-dev
And then the error will magically disappear.