Was getting a weird test failure for Visual C++ 11,
BOOST_NO_CXX11_HDR_TUPLE is defined, so the code doesn't support
std::tuple, but BOOST_UNORDERED_HAVE_PIECEWISE_CONSTRUCT was also
true, and so there are functions for constructing using
std::piecewise_construct/std::tuple, which don't work.
So, I'm assuming that if BOOST_UNORDERED_HAVE_PIECEWISE_CONSTRUCT is true,
then there must be a std::tuple. I guess it doesn't have full C++11 support,
which is why BOOST_NO_CXX11_HDR_TUPLE is defined, but it appears to be
good enough for us. If not, this will break things.