Fix windows build

Catch up to the split of preauth_plugin.h into client and
kdc specific portions.  While here, use copy's /y flag to suppress
an override-confirmation prompt (though we do not list any dependencies
for this target at the moment, so no such prompt will be generated).

Do not disable library finalizers for windows, erroneously disabled
in 4538146e1452e0966164119cefb0804993ce6bbb.