[boost] [config] C++0x macros

There was some past discussion of changing the C++0x BOOST_HAS_* macros to BOOST_NO_*. Several participants thought that was better for the long run. Who needs to do what to make this happen? The motivation for asking is that I'm starting to update some Boost code that could benefit from C++0x features and so would like to start using these macros. Also, a C++03 workaround for the lack of the C++0x keyword "constexpr" is to simply omit it. Should we have a BOOST_CONSTEXPR macro defined as "constexpr" for platforms that support constexpr, otherwise as null? --Beman