[boost] [config] BOOST_NO_MEMBER_FUNCTION_SPECIALIZATIONS

AFAICT, the confing macro BOOST_NO_MEMBER_FUNCTION_SPECIALIZATIONS is not used anywhere outside the config tests that test for this feature! Should we retire old workarounds like this, that are no longer tested for? Or do they add value to the config library, supporting 'customer' code relying on config for its own feature detection / workarounds? -- AlisdairM