Require: No Such File to Load — Mkmf (LoadError)

While attempting to install a ruby gem you may get this error:

Building native extensions.  This could take a while...
ERROR:  Error installing rails:
    ERROR: Failed to build gem native extension.

        /usr/bin/ruby1.8 extconf.rb
extconf.rb:36:in `require': no such file to load -- mkmf (LoadError)
    from extconf.rb:36

To resvole this:

apt-get install ruby1.9.1-dev