gem fails to install

[Thufir]

When trying to install buildr the result is this error:
ERROR: Failed to build gem native extension.

Is this the correct way to install gems?  So far as I know I'm doing it 
correctly.

The PDF directions for this gem, buildr, discussed logging in as root so 
that was my approach.


root at arrakis:~# echo $JAVA_HOME
/usr/lib/jvm/java-6-sun/jre/bin/java
root at arrakis:~# 
root at arrakis:~# 
root at arrakis:~# 
root at arrakis:~# 
root at arrakis:~# 
root at arrakis:~# gem install buildr
Bulk updating Gem source index for: http://gems.rubyforge.org
Install required dependency rake? [Yn]  y
Install required dependency facets? [Yn]  y
Install required dependency builder? [Yn]  y
Install required dependency net-ssh? [Yn]  y
Install required dependency needle? [Yn]  y
Install required dependency net-sftp? [Yn]  y
Install required dependency rubyzip? [Yn]  y
Install required dependency highline? [Yn]  y
Install required dependency Antwrap? [Yn]  y
Select which gem to install for your platform (i486-linux)
 1. Antwrap 0.6.0 (ruby)
 2. Antwrap 0.6.0 (java)
 3. Skip this gem
 4. Cancel installation
> 1
Install required dependency rjb? [Yn]  y
Select which gem to install for your platform (i486-linux)
 1. rjb 1.1.2 (x86-mswin32-60)
 2. rjb 1.1.2 (ruby)
 3. rjb 1.1.1 (x86-mswin32-60)
 4. rjb 1.1.1 (ruby)
 5. Skip this gem
 6. Cancel installation
> 2
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 buildr
extconf.rb:6:in `require': no such file to load -- mkmf (LoadError)
        from extconf.rb:6


Gem files will remain installed in /var/lib/gems/1.8/gems/rjb-1.1.2 for 
inspection.
Results logged to /var/lib/gems/1.8/gems/rjb-1.1.2/ext/gem_make.out
root at arrakis:~# 
root at arrakis:~# 




thanks,

Thufir