To fix compilation on MacOS.
I think actually we want to use this pervasively in our tests
on Linux; it doesn't really matter when run inside a transient
container, but `PDEATHSIG` is useful for persistent containers (e.g.)
toolbox and when running outside of a pid namespace, e.g. on a host
system shell directly or in systemd.
Signed-off-by: Colin Walters <walters@verbum.org>
