SUSv2 mandates that <fcntl.h> provide both F_SETFD and FD_CLOEXEC,
so it seems pretty unlikely that any platforms remain without those.
Remove the #ifdef-ery installed by commit 7627b91c to see if the
buildfarm agrees.

Discussion: https://postgr.es/m/21444.1492798101@sss.pgh.pa.us