Surround with a multiple inclusion guard, and use the modern C++ [[maybe_unused]] thing only when available. Change-Id: Id1eebd65edb6846d1551b56e208917c2c1f6b34d Reviewed-on: https://gerrit.libreoffice.org/c/core/+/196681 Tested-by: Tor Lillqvist <tml@iki.fi> Reviewed-by: Tor Lillqvist <tml@iki.fi> Reviewed-by: Miklos Vajna <vmiklos@collabora.com> Reviewed-on: https://gerrit.libreoffice.org/c/core/+/197541 Tested-by: Jenkins