Because we do not define __unused, as on GNU systems Linux and glibc
providing conflicting symbols, declarations for imported functions that
use that attribute cause build failures when used.
Ref: https://bugs.debian.org/1083196Fixes: #34
This is a non-portable header, and we cannot expect it to be provided by
the system libc (e.g. musl). We just need and rely on declaration that
we have defined ourselves in our own <bsd/sys/cdefs.h>. So we switch to
only ever assume that.
Fixes: https://bugs.freedesktop.org/105281
