[boost] [config] gcc and <unordered_set> vs <ext/hash_set>

Folks, Dave A. asked for a config macro to detect the presence of std conforming <unordered_set> and <unordered_map>. However, I've run into an unexpected issue: it appears that with gcc-4.3.x you cannot include both <unordered_set> and <ext/hash_set> as attempting to do so leads to endless errors inside the <ext/*> headers :-( Including either one of the headers is just fine though. So... how should we handled this? Not define BOOST_HAS_HASH when <unordered_set> *is* available? As far as I can tell the only library effected by this change would be Boost.Serialization which would lose has_set support in gcc's c++0x mode. How do folks feel about that? Regards, John.