Fix broken links in documentation.

Some documentation links currently point to pages that HP retired along with the rest of the SGI STL website. These links now point to the Boost mirror site.

concept_check.htm

@@ -220,7 +220,7 @@ bad_error_eg.cpp:8:   instantiated from here
 
   <p>In this case, the fundamental error is
   that <tt>std:complex&lt;float&gt;</tt> does not model the <a href=
-  "http://www.sgi.com/tech/stl/LessThanComparable.html">LessThanComparable</a>
+  "http://www.boost.org/sgi/stl/LessThanComparable.html">LessThanComparable</a>
   concept.  Unfortunately, there is nothing in the error message to
   indicate that to the user.</p>
 
@@ -232,7 +232,7 @@ bad_error_eg.cpp:8:   instantiated from here
   <ol>
     <li>There is no textual correlation between the error message and the
     documented requirements for <tt>std::stable_sort()</tt> and for <a href=
-    "http://www.sgi.com/tech/stl/LessThanComparable.html">LessThanComparable</a>.</li>
+    "http://www.boost.org/sgi/stl/LessThanComparable.html">LessThanComparable</a>.</li>
 
     <li>The error message is overly long, listing functions internal
     to the STL (e.g. <code>__insertion_sort</code>) that the user
@@ -268,7 +268,7 @@ boost/concept_check.hpp:236: error: no match for ‘operator&lt;’ in ‘((boos
   <ul>
     <li>The message refers explicitly to concepts that the user can look up
     in the STL documentation (<a href=
-    "http://www.sgi.com/tech/stl/LessThanComparable.html">LessThanComparable</a>).</li>
+    "http://www.boost.org/sgi/stl/LessThanComparable.html">LessThanComparable</a>).</li>
 
     <li>The error message is now much shorter and does not reveal
     internal STL functions, nor indeed does it even point