[ccing bug-gnulib; this is about <https://bugs.gnu.org/42095>.]

> The latest changes made this code run even in the MinGW build, not
> sure why (perhaps because 'getrandom' uses 'open'?).

That's part of it. It's also needed for recent changes to the getloadavg module.
And I see that the at-internal module also depends on the 'open' module, I
presume because lib/openat-proc.c opens /proc/self/fd/NNN/, but this surely
won't work in mingw.

As I understand it the basic issue here is that we want O_CLOEXEC support even
on platforms that lack it, and we don't want all the calling software to play
games with "#ifdef O_CLOEXEC" and the like.

I attempted to work around this particular problem by installing the attached
workaround into Gnulib and updating Emacs accordingly. Please give it a try.
Perhaps this should be fixed more systematically in Gnulib instead of worked
around, but I suppose that might entail some merging between Emacs's and
Gnulib's ways of dealing with file names under MS-Windows and I'll leave it to
the MS-Windows experts to figure out how to do that, or whether they want to do
it at all.