All systems must implement it by now. If there's any system still
without it, that means it has no IPv6 support, so they can disable
QtNetwork entirely.
[ChangeLog][Deprecation Notice] Starting with Qt 5.10, IPv6 support is
mandatory for all platforms. Systems without proper IPv6 support, such
as the getaddrinfo() function or the proper socket address structures,
will not be able to build QtNetwork anymore.
Change-Id: I3868166e5efc45538544fffd14d8c28046f9191b
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
3fe74b76fd