configure.ac: error if --with-winidn and target OS too old

[jay]

• If the minimum Windows target OS is pre-Vista (_WIN32_WINNT < 0x600) then error.

• Stop comparing against WINVER when making the minimum OS determinations (ie check only _WIN32_WINNT instead of both since that's the one that matters).

Prior to this change, if --with-winidn was used in such a case configure would change the minimum target OS to Vista (0x600) so the build would compile. That was done primarily as a workaround for original MinGW which we no longer support.

Bug: #12606 (comment)
Reported-by: Viktor Szakats

Closes #xxxxx

[vszakats]

Thinking about this, if we require Vista, we'd need some additional tweaks for consistency. In lib/idn.c we have logic to declare these WinIDN APIs ourselves when building for < 0x0600. (Which is kind of valid, since WinIDN could also work on XP, though it needed normaliz.dll, which used to be a separate download, but Microsoft deleted that download a long time ago. So in practice it became problematic.)

Anyway, if we require Vista for IDN, the lib/idn.c trick is no longer necessary and it'd probably be better to replace it with something like this in lib/curl-setup.h and CMake could also use an update:

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3a6ae3d42..332c08186 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -841,13 +841,6 @@ else()
   set(HAVE_LIBIDN2 OFF)
 endif()
 
-if(WIN32)
-  option(USE_WIN32_IDN "Use WinIDN for IDN support" OFF)
-  if(USE_WIN32_IDN)
-    list(APPEND CURL_LIBS "normaliz")
-  endif()
-endif()
-
 #libpsl
 option(CURL_USE_LIBPSL "Use libPSL" ON)
 mark_as_advanced(CURL_USE_LIBPSL)
@@ -1085,6 +1078,14 @@ if(WIN32)
       unset(HAVE_INET_PTON CACHE)
     endif()
   endif()
+
+  option(USE_WIN32_IDN "Use WinIDN for IDN support" OFF)
+  if(USE_WIN32_IDN)
+    if(HAVE_WIN32_WINNT AND HAVE_WIN32_WINNT STRLESS "0x600")
+      message(FATAL_ERROR "WinIDN requires building for Windows Vista or newer.")
+    endif()
+    list(APPEND CURL_LIBS "normaliz")
+  endif()
 endif()
 
 check_include_file_concat("sys/filio.h"      HAVE_SYS_FILIO_H)
diff --git a/lib/curl_setup.h b/lib/curl_setup.h
index 551243e27..794d42058 100644
--- a/lib/curl_setup.h
+++ b/lib/curl_setup.h
@@ -623,6 +623,14 @@
 
 /* ---------------------------------------------------------------- */
 
+#if !defined(_WIN32) && defined(USE_WIN32_IDN)
+#undef USE_WIN32_IDN
+#endif
+
+#if defined(USE_WIN32_IDN) && (_WIN32_WINNT < 0x0600)
+#error "WinIDN requires building for Windows Vista or newer."
+#endif
+
 #if defined(HAVE_LIBIDN2) && defined(HAVE_IDN2_H) && !defined(USE_WIN32_IDN)
 /* The lib and header are present */
 #define USE_LIBIDN2
diff --git a/lib/idn.c b/lib/idn.c
index 81a177f8c..c4167db18 100644
--- a/lib/idn.c
+++ b/lib/idn.c
@@ -51,20 +51,6 @@
 #include "memdebug.h"
 
 #ifdef USE_WIN32_IDN
-/* using Windows kernel32 and normaliz libraries. */
-
-#if !defined(_WIN32_WINNT) || _WIN32_WINNT < 0x600
-WINBASEAPI int WINAPI IdnToAscii(DWORD dwFlags,
-                                 const WCHAR *lpUnicodeCharStr,
-                                 int cchUnicodeChar,
-                                 WCHAR *lpASCIICharStr,
-                                 int cchASCIIChar);
-WINBASEAPI int WINAPI IdnToUnicode(DWORD dwFlags,
-                                   const WCHAR *lpASCIICharStr,
-                                   int cchASCIIChar,
-                                   WCHAR *lpUnicodeCharStr,
-                                   int cchUnicodeChar);
-#endif
 
 #define IDN_MAX_LENGTH 255

Edits:

• autotools' --with-winidn=PATH option is also obsolete in this case (because IDN is part of the SDK for Vista)
• autotools' normaliz checker is also redundant because it's always present with Vista.
• we might consider enabling WinIDN by default if libidn2 isn't enabled and targeting Windows.

If want to keep supporting WinIDN for XP, maybe the autotools detection could probe building with IdnToAscii() with normaliz lib, but that's a little shaky because an XP target env might not have the DLL.