bug#50666: 28.0.50; Fix native compilation on Cygwin

[Eli Zaretskii]

> Date: Thu, 28 Oct 2021 18:22:28 -0400
> From: Ken Brown <kbrown@cornell.edu>
> Cc: Stromeko@nexgo.de, 50666@debbugs.gnu.org, akrl@sdf.org
> 
> 32-bit Cygwin has just been demoted to a third-class citizen.  Cygwin 3.3.0 
> was 
> released this morning, with a deprecation notice that it is the last major 
> version supporting 32-bit installations.  In view of this, I don't want to 
> put 
> any energy into supporting native compilation on 32-bit Cygwin, and I doubt 
> if 
> Achim does either.
> 
> Eli, what do you think of the attached (assuming Achim agrees)?

Assuming this decision of the Cygwin developers is final, I don't
mind.  But why do we want to depend on an environment variable for
enabling the 32-bit build? why not a configure-time command-line
option?