Installing Datamapper on Ubuntu 9.04, I got this mysterious error:
$ sudo gem install do_mysql
Building native extensions.  This could take a while…
ERROR:  Error installing do_mysql:
ERROR: Failed to build gem native extension.
/usr/bin/ruby1.8 extconf.rb install do_mysql
extconf.rb:3:in `require’: no such file to load — mkmf (LoadError)
from extconf.rb:3

After much googling and trying other solutions like installing libmysqlclient16-dev, I stumbled upon [...]