[PATCH] Enable libstdc++ numeric conversions on Cygwin

[Jennifer Yao]

> I just finished running the testsuite on x86_64-pc-cygwin for
> Jonathan's latest patch and compared the results against an older
> (about two months old) run, and so far I'm not seeing any regressions.
> Granted, this is strictly preliminary; I'm currently re-running the
> testsuite on the up-to-date, unpatched build tree to see if there are
> any differences. I'll update when I have more information.

Finally finished testing the patch that was committed to trunk on Nov.
13. No regressions to report for x86_64-pc-cygwin.