lorenzo@ubuntu:~$ sudo gem1.8 install daemons gem_plugin mongrel mongrel_cluster --include-dependencies
INFO: `gem install -y` is now default and will be removed
INFO: use --ignore-dependencies to install only the gems you list
Successfully installed daemons-1.1.4
Successfully installed gem_plugin-0.2.3
Building native extensions. This could take a while...
ERROR: Error installing mongrel:
ERROR: Failed to build gem native extension.
/usr/bin/ruby1.8 extconf.rb
extconf.rb:8:in `require': no such file to load -- mkmf (LoadError)
from extconf.rb:8
Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/fastthread-1.0.7 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/fastthread-1.0.7/ext/fastthread/gem_make.out
Building native extensions. This could take a while...
ERROR: Error installing mongrel_cluster:
ERROR: Failed to build gem native extension.
/usr/bin/ruby1.8 extconf.rb
extconf.rb:8:in `require': no such file to load -- mkmf (LoadError)
from extconf.rb:8
Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/fastthread-1.0.7 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/fastthread-1.0.7/ext/fastthread/gem_make.out
2 gems installed
Installing ri documentation for daemons-1.1.4...
Installing ri documentation for gem_plugin-0.2.3...
Installing RDoc documentation for daemons-1.1.4...
Installing RDoc documentation for gem_plugin-0.2.3...
The solution is to install the ruby-dev package:
lorenzo@ubuntu:~$ sudo apt-get install ruby-dev