Revert @endragor's fix for Windows
Revert @endragor's fix for Windows

_mkgmtime is not supported by mingw because of the older msvcrt
versions. Fortunately, mktime implementation in Windows is just broken
enough to make the initial implementation work...

As far as I can tell, this works with both *nix-like platforms
and Win, and fixes #4690

russpowers/Nim,tulayang/Nim,dom96/Nim,russpowers/Nim,tulayang/Nim,dom96/Nim,tulayang/Nim,dom96/Nim,tulayang/Nim,dom96/Nim,russpowers/Nim,russpowers/Nim,tulayang/Nim,dom96/Nim,dom96/Nim,dom96/Nim,russpowers/Nim,russpowers/Nim,tulayang/Nim,russpowers/Nim