[boost] [config] Variadic template macros in gcc header

Why is there two mutually exclusive macros indicating variadic template support for gcc ? There is the established BOOST_NO_VARIADIC_TEMPLATES which says that any compiler does not have variadic template support. Then I find that for gcc there is also BOOST_HAS_VARIADIC_TMPL which evidently says that the compiler does have variadic template support, and is in no other compiler header file than the gcc one. Since the two config macros are mutually exclusive in the gcc header file, why is there any need for BOOST_HAS_VARIADIC_TMPL ?