title: `require': no such file to load -- mkmf (LoadError)
body: <p>I was trying to install rails on Ubuntu Natty Narwhal 11.04, using ruby1.9.1.</p>
  
  <p>I installed ruby using <code>apt-get install ruby1.9.1-full</code> which contains the dev package. I googled the error and all have suggested I install the 1.9.1-dev which I already have.</p>
  
  <pre><code>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
  
  
  Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/bcrypt-ruby-3.0.1 for inspection.
  Results logged to /usr/lib/ruby/gems/1.8/gems/bcrypt-ruby-3.0.1/ext/mri/gem_make.out
  </code></pre>