windows: bump minimum to Vista (from XP)

After this patch curl requires targeting Vista or newer, and a toolchain
with Vista support.

Supported MSVC compilers (VS2010+) all support Vista:
- VS2012+ target Win8 (or later) by default.
- VS2010 targets Win7 by default.

Supported mingw-w64 versions (v3+) all support Vista:
- mingw-w64 v9+ target Win10 by default.
- mingw-w64 v8 and older target Server 2003 (~XP) by default.
  After this patch it may be necessary to override the default Windows
  target version to Vista (or newer) via:
  autotools: `CPPFLAGS=-D_WIN32_WINNT=0x0600`
  cmake: `-DCURL_TARGET_WINDOWS_VERSION=0x0600`
- mingw-w64 v6+ allow changing the default at toolchain build-time.

Notes:
- For non-MSVC, non-mingw-w64 toolchains, `if_nametoindex` needs to be
  allowlisted in `curl_setup.h`, if they do support it.

Fixes #17985 (discussion)
Closes #18009

lib/curl_setup.h

@@ -156,6 +156,18 @@
 
 #endif /* HAVE_CONFIG_H */
 
+#ifdef _WIN32
+#  if defined(_WIN32_WINNT) && (_WIN32_WINNT < 0x0600)
+#    error The minimum build target is Windows Vista (0x0600)
+#  endif
+
+#  if !defined(CURL_WINDOWS_UWP) && (defined(_MSC_VER) || defined(__MINGW32__))
+#    ifndef HAVE_IF_NAMETOINDEX
+#    define HAVE_IF_NAMETOINDEX
+#    endif
+#  endif
+#endif
+
 /* ================================================================ */
 /* Definition of preprocessor macros/symbols which modify compiler  */
 /* behavior or generated code characteristics must be done here,    */