configure: set classic mingw minimum OS version to XP

- If the user has not specified a minimum OS version (via WINVER or
  _WIN32_WINNT macros) then set it to Windows XP.

Prior to this change classic MinGW defaulted the minimum OS version
to Windows NT 4.0 which is way too old. At least Windows XP is needed
for getaddrinfo (which resolves hostnames to IPv6 addresses).

Ref: https://github.com/curl/curl/issues/7483#issuecomment-891597034

Closes https://github.com/curl/curl/pull/7581

configure.ac

@@ -446,6 +446,32 @@ else
 fi
 AM_CONDITIONAL(BUILD_UNITTESTS, test x$want_unittests = xyes)
 
+# For original MinGW (ie not MinGW-w64) define the Windows minimum supported OS
+# version to Windows XP (0x501) if it hasn't already been defined by the user.
+# Without this override original MinGW defaults the version to Windows NT 4.0.
+# Note original MinGW sets _WIN32_WINNT if not defined to whatever WINVER is.
+case $host in
+  *-*-mingw32*)
+    AC_MSG_CHECKING([if MinGW minimum supported OS should be set to XP])
+    AC_COMPILE_IFELSE([
+      AC_LANG_PROGRAM([[
+#include <_mingw.h>
+      ]],[[
+#if defined(__MINGW64_VERSION_MAJOR) || \
+    defined(WINVER) || \
+    defined(_WIN32_WINNT)
+#error
+#endif
+      ]])
+    ],[
+      CPPFLAGS="$CPPFLAGS -DWINVER=0x501"
+      AC_MSG_RESULT([yes])
+    ],[
+      AC_MSG_RESULT([no])
+    ])
+    ;;
+esac
+
 dnl **********************************************************************
 dnl Compilation based checks should not be done before this point.
 dnl **********************************************************************
@@ -2309,11 +2335,12 @@ esac
 
 if test "$want_winidn" = "yes"; then
   dnl winidn library support has been requested
+  clean_CFLAGS="$CFLAGS"
   clean_CPPFLAGS="$CPPFLAGS"
   clean_LDFLAGS="$LDFLAGS"
   clean_LIBS="$LIBS"
   WINIDN_LIBS="-lnormaliz"
-  WINIDN_CPPFLAGS="-DWINVER=0x0600"
+  WINIDN_CPPFLAGS=""
   #
   if test "$want_winidn_path" != "default"; then
     dnl path has been specified
@@ -2323,6 +2350,24 @@ if test "$want_winidn" = "yes"; then
     WINIDN_DIR="$want_winidn_path/lib$libsuff"
   fi
   #
+  dnl WinIDN requires a minimum supported OS version of at least Vista (0x0600)
+  AC_COMPILE_IFELSE([
+    AC_LANG_PROGRAM([[
+      #include <windows.h>
+    ]],[[
+      #if (WINVER < 0x600) && (_WIN32_WINNT < 0x600)
+      #error
+      #endif
+    ]])
+  ],[
+  ],[
+     CFLAGS=`echo $CFLAGS | $SED -e 's/-DWINVER=[[^ ]]*//g'`
+     CFLAGS=`echo $CFLAGS | $SED -e 's/-D_WIN32_WINNT=[[^ ]]*//g'`
+     CPPFLAGS=`echo $CPPFLAGS | $SED -e 's/-DWINVER=[[^ ]]*//g'`
+     CPPFLAGS=`echo $CPPFLAGS | $SED -e 's/-D_WIN32_WINNT=[[^ ]]*//g'`
+     WINIDN_CPPFLAGS="$WINIDN_CPPFLAGS -DWINVER=0x0600"
+  ])
+  #
   CPPFLAGS="$CPPFLAGS $WINIDN_CPPFLAGS"
   LDFLAGS="$LDFLAGS $WINIDN_LDFLAGS"
   LIBS="$WINIDN_LIBS $LIBS"
@@ -2349,6 +2394,7 @@ if test "$want_winidn" = "yes"; then
     curl_idn_msg="enabled (Windows-native)"
   else
     AC_MSG_WARN([Cannot find libraries for IDN support: IDN disabled])
+    CFLAGS="$clean_CFLAGS"
     CPPFLAGS="$clean_CPPFLAGS"
     LDFLAGS="$clean_LDFLAGS"
     LIBS="$clean_LIBS"