• autotools: fix auto-detection on the Windows platform.
  It was mis-detected when targeting Windows XP/2003 64-bit.
  It was permanently undetected when building for Windows 32-bit.

  lib/url.c: In function 'zonefrom_url':
  lib/url.c:1802:18: error: implicit declaration of function 'if_nametoindex' [-Wimplicit-function-declaration]
   1802 |       scopeidx = if_nametoindex(zoneid);
        |                  ^~~~~~~~~~~~~~
  lib/url.c:1802:18: error: nested extern declaration of 'if_nametoindex' [-Werror=nested-externs]
  

  Ref: https://github.com/curl/curl/actions/runs/16405598782/job/46351023138?pr=17982#step:10:29

  Reported-by: LoRd_MuldeR
  Fixes cURL 8.15.0 fails to build for 64-Bit Windows XP target (mingw-w64), but not for 32-Bit Windows target #17979

  Without this patch the workaround for the 8.15.0 release is:
  export ac_cv_func_if_nametoindex=0 for Windows XP/2003 64-bit.

  Background: Checking for the if_nametoindex() function via
  AC_CHECK_FUNCS() (autotools) or check_function_exists() (cmake) do
  not work on Windows, for two reasons:

  • the function may be disabled at compile-time in Windows headers
    when targeting old Windows versions (XP or WS2003 in curl context)
    via _WIN32_WINNT. But it's always present in the system implib
    iphlpapi where these checks are looking.
  • for 32-bit Windows the function signature in the implib requires
    a 4-byte argument, while these checks always use no arguments,
    making them always fail.

• cmake: call if_nametoindex dynamically with mingw-w64 v1.0.
  This mingw-w64 version lacks prototype and implib entry for it.

• cmake: add auto-detection for Windows and use as a fallback for
  non-pre-fill cases.

• cmake: disable pre-fill with _CURL_PREFILL=OFF. (for testing)

• cmake: disable pre-fill for untested compilers. (i.e. non-MSVC,
  non-mingw64)

• GHA/windows: make an autotools job build for Windows XP.

Follow-up to 0d71b18 #17413