bug#50666: 28.0.50; Fix native compilation on Cygwin

[Eli Zaretskii]

> From: Ken Brown <kbrown@cornell.edu>
> Date: Sat, 18 Sep 2021 16:46:42 -0400
> 
> In a followup to this message, I'll submit a patch that does this
> ephemeral rebase and fixes the build problem.

Thanks.

> Note: The build will not actually be convenient to use on 32-bit Cygwin, 
> because 
> sooner or later the *.eln files in ~/.emacs.d/eln-cache will also need to be 
> rebased.  I hope to address this in future patches.

Why would they need to be rebased, and why doesn't your solution
handle those *.eln files as well?