On macOS the relevant functions require no special library, but elsewhere
we need to pull in libdns_sd.

Back-patch to supported branches.  No docs change since the docs do not
suggest that this is a Mac-only feature.

Luke Lonergan

Discussion: https://postgr.es/m/2D8331C5-D64F-44C1-8717-63EDC6EAF7EB@brightforge.com