FreeBSD: use _XOPEN_SOURCE=600 instead of 500

New systemd-code uses sockaddr_in6, which in FreeBSD is only defined if
_POSIX_C_SOURCE is 200112L. With _XOPEN_SOURCE set to 500, _POSIX_C_SOURCE
is forced to 199506.
Note: systemd does not exist on FreeBSD afaik, but this fixes overall build.
Merge remote-tracking branch 'origin/master' into darwin

    merge: FreeBSD: use _XOPEN_SOURCE=600 instead of 500