Autoconf's AC_CHECK_DECLS() always defines HAVE_DECL_whatever
as 1 or 0, but some of the entries in msvc/Solution.pm showed
such symbols as "undef" instead of 0.  Fix that for consistency.
There's no live bug in current usages AFAICS, but it's not hard
to imagine one creeping in if more-complex #if tests get added.

Back-patch to v13, which is as far back as Solution.pm contains
this data.  The inconsistency still exists in the manually-filled
pg_config_ext.h.win32 files of older branches; but as long as the
problem is only latent, it doesn't seem worth the trouble to
clean things up there.

Discussion: https://postgr.es/m/3185430.1626133592@sss.pgh.pa.us