.. because not all Windows build systems have those symbols, and even
those that do may be missing newer symbols (eg the Windows 7 SDK does
not define _WIN32_WINNT_WIN10).
Those symbols are used in build-time logic to decide which API to use
and prior to this change if the symbols were missing it would have
resulted in deprecated API being used when more recent functions were
available (eg GetVersionEx used instead of VerifyVersionInfo).
Reported-by: FuccDucc@users.noreply.github.com
Probably fixes https://github.com/curl/curl/issues/4995
Closes https://github.com/curl/curl/pull/5057
