NetBSD build fix
On NetBSD, the shm_unlink() etc. functions are in librt, not in libc like on FreeBSD or OpenBSD; see https://man.netbsd.org/shm_unlink.3
Please apply the following patch to fix the build:
--- sys/shm/meson.build.orig 2021-09-08 19:03:40.991253900 +0000
+++ sys/shm/meson.build
@@ -11,7 +11,7 @@ if get_option('shm').disabled()
endif
shm_deps = []
-if ['darwin', 'ios'].contains(host_system) or host_system.endswith('bsd')
+if ['darwin', 'ios', 'freebsd', 'openbsd'].contains(host_system)
rt_dep = []
shm_enabled = true
else