[committed] libstdc++: Test errno macros directly, not via autoconf [PR 93151]

[Eric Botcazou]

> Oops, and I also forgot to update the other headers that were using
> those autoconf-generated macros!
> 
> Fixed by this patch. Tested on mingw-w64, pushed to trunk.

The original patch was apparently backported onto the gcc-10 branch, but not 
this follow-up fix, so Windows builds are now broken on the branch.

-- 
Eric Botcazou