[v3] -fno-exceptions cleanups

[Benjamin Kosnik]

> The __throw_future_error function needs to be guarded by
> 
> #if defined(_GLIBCXX_HAS_GTHREADS) &&
> defined(_GLIBCXX_USE_C99_STDINT_TR1) \ &&
> defined(_GLIBCXX_ATOMIC_BUILTINS_4)
> 
> or future_error and future_errc need to be moved outside the guard and
> defined unconditionally.

Ooops, sorry. Of course it wasn't the obvious straightforward
thing.... yes, I will do this last one. I also want to try and implement
a base class for all the types in this header... will do that later.

-benjamin