Otherwise, dbus doesn't compile on FreeBSD if the GLib-based tests are enabled (which suggests that no FreeBSD user has run those tests successfully).
We already include <netinet/in.h> in other places with no conditions or checks other than "is Unix", so apparently it's portable enough that specifically testing for its presence is not necessary. POSIX requires it to exist.
Also nominated for dbus-1.12 branch, because it'll make it easier for the (possibly hypothetical) people who test dbus on non-Linux Unix.