[boost] [preprocessor] BOOST_PP_OVERLOAD not defined if variadic macros not supported.

I just found out the hard way that BOOST_PP_OVERLOAD is "silently" not defined if variadic macros are not supported , ie in c++03. This led to quite a frustrating session of debugging on the premise that there's something wrong with my include paths/environment variables. It would be very nice if BOOST_PP_OVERLOAD errored explicitly if variadic macros were not supported by the compiler, or at lease have a gentle reminder to the reader in BOOST_PP_OVERLOAD documentation (and, where appropriate, in other places) that variadic macros are only standard in c++11 and may not be supported on all compilers, hence one might get a BOOST_PP_OVERLOAD undeclared error. Mostafa