On Sat, 27 Mar 2021 16:58:48 +0100, Marco Atzeri
> I used your cygport as base for 3.20.0-1, plus an additional patch
> https://sourceware.org/pipermail/cygwin-patches/2021q1/011303.html

I've found another confusion between _WIN32 and __CYGWIN__ in 3.20.0 [1].
[1]: https://gitlab.kitware.com/cmake/cmake/-/blob/v3.20.0/Source/kwsys/SystemTools.cxx#L3930-3961

A patch attached will resolve it.

Regards,

Lem