[boost] [TypeTraits] Patch: improving detectability of clang's intrinsics

Attached a patch to improve detectability of `__is_pod` and `__is_empty` intrinsics in clang. This is done by newly introduced `__has_feature(__is_pod__)` and `__has_feature(__is_empty__)` macros. (Clang trunk now accepts feature names with preceding and following double underscores.) Regards, Michel