[patch PR libstdc++/51135]: Fix [4.7 Regression] SIGSEGV during exception cleanup on win32

[Paolo Carlini]

Hi,
> I think I understood that pretty well and wrote therefore that
> configure-stuff should happen in configure part.  Anyway here is my
> updated patch for it.
Patch looks great to me, thanks for persisting. Unless somebody objects 
to it over the next, say 24h, please commit it and resolve the PR.

Thanks,
Paolo.