title: Failed to build gem native extension — Rails install
body: <p>I have been installing Ruby on Rails based on the instruction from edX online class.</p>
  
  <p>Here is a list of terminal commands, and, where relevant, corresponding output:</p>
  
  <pre class="lang-none prettyprint-override"><code>$ sudo apt-get install ruby1.9.1
  </code></pre>
  
  <p>installation successful</p>
  
  <pre class="lang-none prettyprint-override"><code>$ wget http://production.cf.rubygems.org/rubygems/rubygems-1.8.24.tgz
  $ tar zxvf ruby-1.8.24.tgz
  $ cd ruby-1..8.24
  $ sudo ruby setup.rb
  RubyGems 1.8.24 installed
  
  == 1.8.24 / 2012-04-27
  
  * 1 bug fix:
  
    * Install the .pem files properly. Fixes #320
    * Remove OpenSSL dependency from the http code path
  
  
  ------------------------------------------------------------------------------
  
  RubyGems installed the following executables:
      /usr/bin/gem1.9.1
  </code></pre>
  
  <p>Up to here, everything looked fine, but the I tried to install rails and failed:</p>
  
  <pre class="lang-none prettyprint-override"><code>$ sudo gem install rails
  Fetching: i18n-0.6.1.gem (100%)
  Fetching: multi_json-1.3.6.gem (100%)
  Fetching: activesupport-3.2.8.gem (100%)
  Fetching: builder-3.0.3.gem (100%)
  Fetching: activemodel-3.2.8.gem (100%)
  Fetching: rack-1.4.1.gem (100%)
  Fetching: rack-cache-1.2.gem (100%)
  Fetching: rack-test-0.6.1.gem (100%)
  Fetching: journey-1.0.4.gem (100%)
  Fetching: hike-1.2.1.gem (100%)
  Fetching: tilt-1.3.3.gem (100%)
  Fetching: sprockets-2.1.3.gem (100%)
  Fetching: erubis-2.7.0.gem (100%)
  Fetching: actionpack-3.2.8.gem (100%)
  Fetching: arel-3.0.2.gem (100%)
  Fetching: tzinfo-0.3.33.gem (100%)
  Fetching: activerecord-3.2.8.gem (100%)
  Fetching: activeresource-3.2.8.gem (100%)
  Fetching: mime-types-1.19.gem (100%)
  Fetching: polyglot-0.3.3.gem (100%)
  Fetching: treetop-1.4.10.gem (100%)
  Fetching: mail-2.4.4.gem (100%)
  Fetching: actionmailer-3.2.8.gem (100%)
  Fetching: rake-0.9.2.2.gem (100%)
  Fetching: rack-ssl-1.3.2.gem (100%)
  Fetching: thor-0.16.0.gem (100%)
  Fetching: json-1.7.5.gem (100%)
  Building native extensions.  This could take a while...
  ERROR:  Error installing rails:
      ERROR: Failed to build gem native extension.
  
          /usr/bin/ruby1.9.1 extconf.rb
  /usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
      from /usr/local/lib/site_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 /usr/lib/ruby/gems/1.9.1/gems/json-1.7.5 for inspection.
  Results logged to /usr/lib/ruby/gems/1.9.1/gems/json-1.7.5/ext/json/ext/generator/gem_make.out
  </code></pre>