Don't support D-Bus sniffing on OS X.

See freedesktop.org bug 74029:

	https://bugs.freedesktop.org/show_bug.cgi?id=74029

for details.
resolve a Clang warning in can_read_linux()

./pcap-can-linux.c:246:18: warning: comparison of unsigned expression < 0 is
always false [-Wtautological-compare]
if (pkth.caplen < 0)
~~~~~~~~~~~ ^ ~