future: Fix build with libc++'s non-trivially-constructible  std::tuple<>
future: Fix build with libc++'s non-trivially-constructible  std::tuple<>

This adds a hack for std::tuple<> not being trivially constructible in
libc++.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20190502190154.44254-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

syuu1228/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,dreamsxin/seastar,scylladb/seastar,scylladb/seastar,cloudius-systems/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar