We want to set _WIN32_WINNT so that functions will be properly found in
the headers, to target the NT6.1+ (Windows 7+) APIs.

Also improve the checks for if_nametoindex() and if_indextoname() on
Windows as they are supported in Windows Vista+, but they have
to be checked by linking against iphlpapi.lib (or -liphlpapi).  On other
platforms, they are still checked as they were before.

https://bugzilla.gnome.org/show_bug.cgi?id=783270