We used ENABLE_LOGIND for the automake conditional, and HAVE_LOGIND
for the ifdef. That wasn't wrong, but it certainly was confusing.

Also, move the ifdeffery to avoid warning about unused static function
logind_set_wall_message() when logind is disabled.

We defined both $(VERSION) and $(PACKAGE_VERSION) with the same contents.
$(PACKAGE_VERSION) is slightly more descriptive, so settle on that, and
drop the other define.

This makes it easier to use the same generator script as for other
gperf scripts. With automake each gperf file had it's own rule, but
with meson I'm trying to use one script, and this inconsistency made
that harder.