Ubuntu :: Rubygem : Can't Install Mongrel On Lucid Lynx?

I just installed Ubuntu 10.04 on an old desktop computer so that I could use it for Rails. I installed Ruby, rubygems and Rails using

Code: sudo apt-get install and it works fine, but I cannot install Mongrel through

Code:sudo gem install mongrel,it's giving me this block of error message:

Code: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.I tried removing the rubygem 1.3.5 package that came with the Ubuntu repository and downloaded Rubygems 1.3.6 from rubygems.org, but when I tried the same command, it still gives me the same error.