More consistent std::tuple configuration

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.

test/unordered/unnecessary_copy_tests.cpp

@@ -501,7 +501,7 @@ UNORDERED_AUTO_TEST(unnecessary_copy_emplace_map_test)
     COPY_COUNT(0);
     MOVE_COUNT(0);
 
-#if !defined(BOOST_NO_CXX11_HDR_TUPLE) || defined(BOOST_HAS_TR1_TUPLE)
+#if BOOST_UNORDERED_HAS_STD_TUPLE
 
     reset();
     x.emplace(boost::unordered::piecewise_construct,