libblkid: cleanup internal return codes

Signed-off-by: Karel Zak <kzak@redhat.com>
build-sys: use gettext 0.18.3 if possible

We add a little hack for backward compatibility on systems with
older gettext installed

See 0576dbd3 and 81ed13ed.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>