pg_upgrade versus MSVC build scripts

A look at the MSVC buildfarm members shows that they are not building
most of the files added to contrib/pg_upgrade. The reason seems to be
that that module tries to build both an executable program *and* a
shared library, which it does by dint of setting both PROGRAM and
MODULES in its Makefile. Now that is a dirty hack that is nowhere
documented to work, and in fact the pgxs documentation explicitly says
not to do that. It accidentally fails to fail, at the moment, because
of the way pgxs.mk is set up, and because the OBJS variable is only
needed for one of these targets. But the MSVC build scripts aren't
expecting this and evidently disregard PROGRAM after they see MODULES.

We could try to make this a supported build arrangement, but I'm
inclined to think that a cleaner solution is to split out the loadable
module as a separate contrib subdirectory. Thoughts?

regards, tom lane