Na semana passada eu tentei instalar o ruby-debug no meu Ubuntu 9.10 e obtive o erro mostrado abaixo:

andreyq@desenv-ubuntu-9:~$ sudo gem install ruby-debug
[sudo] password for andreyq:
Building native extensions.  This could take a while...
ERROR:  Error installing ruby-debug:
ERROR: Failed to build gem native extension.

/usr/bin/ruby1.8 extconf.rb
extconf.rb:1:in `require': no such file to load -- mkmf (LoadError)
from extconf.rb:1

Gem files will remain installed in /var/lib/gems/1.8/gems/linecache-0.43 for inspection.
Results logged to /var/lib/gems/1.8/gems/linecache-0.43/ext/gem_make.out
andreyq@desenv-ubuntu-9:~$

Depois de algumas pesquisas descobri que precisava ter o pacote

ruby1.8-dev

instalado para satisfazer algumas dependências.

Fiz a instalação do mesmo:

sudo apt-get install ruby1.8-dev

e tentei novamente instalar o ruby-debug:

andreyq@desenv-ubuntu-9:~$ sudo gem install ruby-debug
Building native extensions.  This could take a while...
Building native extensions.  This could take a while...
Successfully installed linecache-0.43
Successfully installed ruby-debug-base-0.10.3
Successfully installed ruby-debug-0.10.3
3 gems installed
Installing ri documentation for linecache-0.43...
Installing ri documentation for ruby-debug-base-0.10.3...
Installing ri documentation for ruby-debug-0.10.3...
Installing RDoc documentation for linecache-0.43...
Installing RDoc documentation for ruby-debug-base-0.10.3...
Installing RDoc documentation for ruby-debug-0.10.3...
andreyq@desenv-ubuntu-9:~$

Ótimo! Desta vez obtive sucesso.

O engraçado é que eu pensava que o pacote ruby1.8-dev só seria necessário se eu fosse alterar o próprio ruby.