windows: target version macro tidy-ups

[vszakats]

• autotools: stop checking for WINVER to detect thread-safety.
  To sync with implementation in easy_lock.h and with cmake.

• replace numeric version with _WIN32_WINNT_VISTA.

• _WIN32_WINNT_VISTA is always defined via setup-win32.h,
  don't check for it.

[vszakats]

I backed off of deleting WINVER checks around the declaration
hack for IDN functions (and the WINVER assignment from curl_setup.h
with it). It's probably unnecessary, passes CI, but it's a murky area.

This could be done safely if requiring Vista, allowing to drop the
IDN declaration hack.