gettext version problem exposed by buildfarm failures on "canary"

I see buildfarm member "canary" (NetBSD 1.6) was recently resurrected,
but is failing to build HEAD, because msgfmt fails like this:

po/es.po:8039: keyword "msgid_plural" unknown
po/es.po:8036: missing `msgstr' section
po/es.po:8039: parse error

This indicates (1) canary needs a newer version of gettext, and
(2) our configure test for a sufficiently new gettext installation
is not in fact correct, because it didn't complain.

I asked about the configure issue some time ago, and was told that
the existing test for bind_textdomain_codeset() in libintl was
sufficient because that was introduced after ngettext(). It could
be that the library's history is different on NetBSD, or perhaps
canary has a recent libintl.so and a not so recent msgfmt.

I doubt it's worth trying to fix this before 8.4 release, but it
should get looked into eventually.

regards, tom lane