title: Fail to install racc with ruby 1.9.x
body: <p>I really have some problems with gem. I'm trying to install racc for ruby so that it can compiler some *.y files. I'm working with my own laptop with Ubuntu. 
  The path of my ruby is </p>
  
  <pre><code>$ which ruby
  /usr/bin/ruby
  </code></pre>
  
  <p>I'm a freshman of ruby. I have tried to install racc with sudo apt-get install racc. But it doesn't work. By the way, what's the difference between apt-get and gem?</p>
  
  <pre><code>$ sudo gem install racc
  Building native extensions.  This could take a while...
  ERROR:  Error installing racc:
      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:3:in `&lt;main&gt;'
  
  
  Gem files will remain installed in /home/jianqing/RubyGems/gems/racc-1.4.9 for inspection.
  Results logged to /home/jianqing/RubyGems/gems/racc-1.4.9/ext/racc/gem_make.out
  </code></pre>
  
  <p>Here is some information about my system.</p>
  
  <pre><code>$ gem env
  RubyGems Environment:
    - RUBYGEMS VERSION: 1.8.23
    - RUBY VERSION: 1.9.3 (2012-04-20 patchlevel 194) [i686-linux]
    - INSTALLATION DIRECTORY: /home/jianqing/RubyGems
    - RUBY EXECUTABLE: /usr/bin/ruby1.9.1
    - EXECUTABLE DIRECTORY: /home/jianqing/RubyGems/bin
    - RUBYGEMS PLATFORMS:
      - ruby
      - x86-linux
    - GEM PATHS:
       - /home/jianqing/RubyGems
       - /home/jianqing/.gem/ruby/1.9.1
       - /var/lib/gems/1.9.1
    - GEM CONFIGURATION:
       - :update_sources =&gt; true
       - :verbose =&gt; true
       - :benchmark =&gt; false
       - :backtrace =&gt; false
       - :bulk_threshold =&gt; 1000
    - REMOTE SOURCES:
       - http://rubygems.org/
  </code></pre>
  
  <p>Any help is appreciated. I almost crazy for it.</p>