title: Why do I get an error installing the JSON gem in Ubuntu?
body: <p>Using Ubuntu, when I run "<code>bundle install</code>" to set up my Rails environment, it throws an error during the JSON gem installation:</p>
  
  <pre><code>Installing json (1.7.3) with native extensions 
  Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
  
  /usr/bin/ruby1.9.1 extconf.rb 
  /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
      from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
      from extconf.rb:1:in `&lt;main&gt;'
  
  Gem files will remain installed in /home/danny/.bundler/tmp/2040/gems/json-1.7.3 for inspection.
  Results logged to /home/danny/.bundler/tmp/2040/gems/json-1.7.3/ext/json/ext/parser/gem_make.out
  An error occured while installing json (1.7.3), and Bundler cannot continue.
  Make sure that `gem install json -v '1.7.3'` succeeds before bundling.
  </code></pre>
  
  <p>When I try installing only the JSON 1.7.3 gem it also gives me an error:</p>
  
  <pre><code>Building native extensions.  This could take a while...
  ERROR:  Error installing json:
      ERROR: Failed to build gem native extension.
  
  /usr/bin/ruby1.9.1 extconf.rb
  /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
      from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
      from extconf.rb:1:in `&lt;main&gt;'
  
  Gem files will remain installed in /var/lib/gems/1.9.1/gems/json-1.7.3 for inspection.
  Results logged to /var/lib/gems/1.9.1/gems/json-1.7.3/ext/json/ext/parser/gem_make.out
  </code></pre>
  
  <p>I've installed RVM and am running Ruby 1.9.3; Why can't it install the JSON gem?</p>