autoreconf: don’t error out when AM_GNU_GETTEXT is used alone.

autoreconf: don’t error out when AM_GNU_GETTEXT is used alone.

Some projects intentionally don't call AM_GNU_GETTEXT_(REQUIRE_)VERSION
because they have all of the gettext infrastructure checked into version
control and they want autoreconf to _not_ run autopoint.  Therefore, make
the “AM_GNU_GETTEXT is used, but not AM_GNU_GETTEXT_(REQUIRE_)VERSION”
and “AM_GNU_GETTEXT_(REQUIRE_)VERSION is used, but not AM_GNU_GETTEXT”
diagnostics be warnings again, as they were in 2.69.

(Technically these diagnostics were always errors, in the sense that
they were reported with a call to Autom4te::Channels::error(), but
prior to 2.70, error() calls made by autoreconf did not cause
autoreconf to exit unsuccessfully, due to an unrelated bug.  So people
came to depend on these diagnostics not being fatal.)