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]
[SVN r24055]
[SVN r17187]
[SVN r8477]
[SVN r8063]