build: allow missing memfd_create on BSDs
- DragonFly (has wlroots), NetBSD (has swc), OpenBSD (no compositors yet) lack both
memfd_create
andSHM_ANON
- FreeBSD (has wlroots + kwin_wayland) added memfd_create(3) in 13.0 but 12.* is supported until 2024-06-30
Edited by Jan Beich