[ruby-on-rails] `require': no such file to load -- mkmf (LoadError)

The problem is still is recursive on Ubuntu 13/04/13.10/14.04

and

sudo apt-get install ruby1.9.1-dev

worked out for me okay. So If you are using Ubuntu 13.04/13.10/14.04 then using this will really come in handy.

This works even if ruby version is 1.9.3. This is because there is no ruby1.9.3-dev available in the Repository...