`require': no such file to load -- mkmf (LoadError)

I was trying to install rails on Ubuntu Natty Narwhal 11.04, using ruby1.9.1. I installed ruby using `apt-get install ruby1.9.1-full` which contains the dev package. I googled the error and all have suggested I install the 1.9.1-dev which I already have. Building native extensions. This could take a while... ERROR: Error installing rails: ERROR: Failed to build gem native extension. /usr/bin/ruby1.8 extconf.rb extconf.rb:36:in `require': no such file to load -- mkmf (LoadError) from extconf.rb:36 Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/bcrypt-ruby-3.0.1 for inspection. Results logged to /usr/lib/ruby/gems/1.8/gems/bcrypt-ruby-3.0.1/ext/mri/gem_make.out