Hi,
According the debian bug report [1], it is not possible to use std::future on armv5 targetting toolchains. This is because libstdc++ will only enable std::future if ATOMIC_INT_LOCK_FREE > 1. There is no LDREX for armv5 and older, so this definition is set to ATOMIC_INT_LOCK_FREE when compiling for ARMv4t or ARMv5.
My impression is that you should be able to use the kernel helpers for atomic operations in lockfree (?) manner, so the ATOMIC_INT_LOCK_FREE definition is probably incorrect on older arm archs then?