Support std::unordered_map/set even if using libstdc++ from GCC.

When passing the appropriate compiler flags, libstdc++ has
supported std::unordered_map and std::unordered_set for a
long time, even without the tr1 namespace. There's no reason to
limit support of non-tr1 containers to libc++ (clang) and MSVC.

(I chose to retain the typo in MSGPACK_HAS_STD_UNOURDERED_MAP as this check is likely going to go away when msgpack-c switches to C++11.)