configure: Disable networking if winsock2.h is available but winsock functions aren't

Previously, if neither of the checks for the closesocket function
succeeded, we still kept winsock2.h and networking in general
enabled.

When targeting the WinRT API subset, the winsock2.h header is
available (making the check for it succeed, giving the impression
that winsock is available), but tests that actually try to use
such a function will fail. In this case, disable the winsock2.h
feature and networking in general, as if the winsock2.h header
test would have failed in the first place.

Signed-off-by: Martin Storsjö <martin@martin.st>
