title: Rubygems do not install on OS X Lion
body: <p>I used to have no problems at all with ruby, gems and all related stuff. But after installing X Code Developer Tools and upgrading to Lion the gems do not install. </p>
  
  <p>I get the following error for every gem. This is the message for sqlite as an example:</p>
  
  <pre><code>Building native extensions.  This could take a while...
  ERROR:  Error installing sqlite3:
  ERROR: Failed to build gem native extension.
  
  /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb
  mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h
  
  Gem files will remain installed in /Library/Ruby/Gems/1.8/gems/sqlite3-1.3.4 for inspection.
  Results logged to /Library/Ruby/Gems/1.8/gems/sqlite3-1.3.4/ext/sqlite3/gem_make.out
  </code></pre>
  
  <p>What went wrong? How to fix it?</p>