libupnp locally overrides the version of _WIN32_WINNT via
configure.ac and autoconfig.h (which is disabled by a vlc contrib
patch when targeting winrt though).

Make sure that autoconfig.h is included everywhere necessary, to
show/hide the relevant parts of winsock headers depending on the
chosen version of _WIN32_WINNT (either default or overridden).

This avoids including the system declarations of these functions if
the default target is >= vista but the build tries to override it
to a lower version.

On the other hand, if the override of _WIN32_WINNT is removed (as
when targeting winrt) and the default is >= vista, skip the local
version of inet_pton.

This should have the same effect as the previous patch, but
evaluated by the preprocessor instead of hardcoding commenting out
of certain source files.