■

これを入れようとしたら
https://rubyforge.org/projects/twitter/

sudo gem install Twitter

Install required dependency hpricot? [Yn]  y
Select which gem to install for your platform (x86_64-linux)
 1. hpricot 0.6 (mswin32)
 2. hpricot 0.6 (jruby)
 3. hpricot 0.6 (ruby)
 4. hpricot 0.5 (ruby)
 5. hpricot 0.5 (mswin32)
 6. Skip this gem
 7. Cancel installation
> 3
Building native extensions.  This could take a while...
ERROR:  While executing gem ... (Gem::Installer::ExtensionBuildError)
    ERROR: Failed to build gem native extension.

ruby extconf.rb install Twitter
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/hpricot-0.6 for inspection.
Results logged to /var/lib/gems/1.8/gems/hpricot-0.6/ext/hpricot_scan/gem_make.out

との事だったので、mkmfとgemでぐぐったら ruby1.8-dev も必要だとか

sudo apt-get install ruby1.8-dev