[v3] proposed fix for libstdc++/49204 causes abi_check failure

[Paolo Carlini]

Hi,
> Here's a different, partial fix for PR 49204, changing the timed 
> waiting functions to return future_status, but not returning 
> future_status::deferred, as that requires the ABI change detailed 
> above. This doesn't change the library ABI, but might still be too big 
> a change for 4.7 at this stage.
I would be in favor of applying the patch.

Paolo.