My version of Ruby on Red Hat Linux is missing mkmf.rb.

$ irb
irb(main):001:0> require 'mkmf'
LoadError: no such file to load -- mkmf
        from (irb):1:in `require'
        from (irb):1

$ ls /usr/lib/ruby/1.8/mkmf.rb
ls: /usr/lib/ruby/1.8/mkmf.rb: No such file or directory

I was able to find reference to a similar bug in Ubuntu where the fix
was to install a ruby-dev package, but I don't think that would apply
to a Linux distribution.

Is mkmf.rb a post-1.8.2 file, or is this a bug?

What's the best fix?

Thanks.