winbuild: Aligned USE_SSPI with other USE_x defines

Renamed external parameter USE_SSPI = yes/no to ENABLE_SSPI = yes/no.
Backwards compatible change: USE_SSPI can still be passed as external
parameter with yes/no value as long as ENABLE_SSPI is not given.

USE_x defines are passed around with true/false values internally,
USE_SSPI is now aligned to this approach, but still accepts external
values yes/no being passed, just like the other defines.