extend oncevar to any POD type

Change-Id: Ia98ee0a67f183e40fb0c38477760124b2c411dc0
Reviewed-on: https://gerrit.libreoffice.org/40564Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>