Allow --with-bonjour to work with non-macOS implementations of Bonjour.
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