Only compile in multicast support on Linux, for the time being.

The calls and structs provided by the underlying platform for
multicast aren't yet exactly well-established POSIX standards, and the
current Linux code is still not considered to be super-robust, and as
such it isn't a good idea to try to duplicate its functionality for
other platforms. We will revisit the issue once the Linux side has
matured a bit.

Change-Id: I630086e22f6980726fd4523cb60dfc8c9fb9b011
1 file changed