bug#50666: 28.0.50; Fix native compilation on Cygwin

[Eli Zaretskii]

> From: ASSI <Stromeko@nexgo.de>
> Date: Fri, 24 Sep 2021 08:11:08 +0200
> Cc: Stromeko@nexgo.de, akrl@sdf.org
> 
> > B- accounts for the absolute filename of
> >    the source file
> 
> In general, is this hardcoded or can we change the hashing strategy for
> Cygwin via some configuration or hook?

It's currently hardcoded.  Why do you need to change it?