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.
Changed BOOST_CONCEPT_WHERE to BOOST_CONCEPT_REQUIRES to be more
consistent with the current C++0x proposal, which now uses a
"requires" keyword in lieu of "where."
Factored GCC workarounds into the BOOST_CONCEPT_USAGE macro.
[SVN r40769]
