under all cases, not just when _PR_INET6_PROBE is defined, because
_PR_INET6_PROBE is only defined when Mac OS X deployment target is 10.1.
Bug 211512: defined IPV6_V6ONLY if it's not defined so that we can build on
Mac OS X 10.1, where this macro is not defined.
git-svn-id: svn://10.0.0.236/trunk@144857 18797224-902f-48f8-a5cc-f745e15eee43