title: How to get headers for unordered_set in gcc v4.1.2?
body: <p>I'd like to use unordered_set without installing Boost. I tried to add <code>--std=gnu++0x</code> but it is not a recognized option.  Does v4.1.2 include unordered_set?  If so, how do I get the header file for it?</p>
  
  <p>This is a Centos 4 machine.</p>