W32: Bump target NT version to 0x601 (7 or newer)

Also remove now-unnecessary if_nametoindex() implementation
(the HAVE_IF_NAMETOINDEX configure check didn't work correctly, it turned out),
which prevents glib from building. if_nametoindex() is available in lphlpapi since
Vista[1], so we don't need a compatibility function for it anymore, as Windows 7
is the new minimally-required version.

[1] https://msdn.microsoft.com/en-us/library/windows/desktop/bb408409(v=vs.85).aspx

https://bugzilla.gnome.org/show_bug.cgi?id=788180