Check if MSG_NOSIGNAL is defined, don't use it otherwise.

Apparently Mac OS X doesn't support MSG_NOSIGNAL,
so dbus_socket_transformer didn't compile.
With this change, it will use 0 instead.

There is a Mac equivalent setting a socket option,
but I'll bother with that later.
http://lists.apple.com/archives/macnetworkprog/2002/Dec/msg00091.html