no such file to load -- mkmf

While checking out Scott Baron’s coverage-tool for Ruby, insurance, I ran into a minor issue on my - by now - fairly beat up Debian installation.

jcop@mental:~$ sudo gem install insurance
Attempting local installation of 'insurance'
Local gem file not found: insurance*.gem
Attempting remote installation of 'insurance'
Updating Gem source index for:
Building native extensions.  This could take a while...
extconf.rb:1:in `require': no such file to load -- mkmf (LoadError)
        from extconf.rb:1

Thankfully, Google led me to the answer on RubyForge. For some reason, mkmf.rb is part of the ruby1.8-dev package, and initially I hadn’t installed that since it is described as

Header files for compiling extension modules for the Ruby 1.8

Ah well. A quick

sudo apt-get install ruby1.8-dev

and everything trotted along happily after that.