I'd prefer that all the changes for such a build be committed at once
together, not bit by bit, and only if you eventually succeed to
produce a working binary using this method.
> I think you're also saying that I should test both HAVE_SELECT &&Yes, at the very least.
> HAVE_SYS_SELECT_H therefore?
But let's delay this until you have gone all
the way to building Emacs using this method. We won't know all of the
implications until you do.