std::future is not available for all architectures, e.g. it is missing
for ARMv5 (soft-float).
The problem is that libstdc++ only enables std::future if
ATOMIC_INT_LOCK_FREE > 1 which is not true for the ARMv5 target.
As the future functionality is not used for much we just ifdef out the
parts from std::future.
Closes: #8
