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

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

Closes #xxxxx

configure.ac

@@ -2468,7 +2468,6 @@ if test "$want_winidn" = "yes"; then
  WINIDN_DIR="$want_winidn_path/lib$libsuff"
  fi
  #
- dnl WinIDN requires a minimum supported OS version of at least Vista (0x0600)
  AC_COMPILE_IFELSE([
  AC_LANG_PROGRAM([[
  #include <windows.h>
@@ -2479,11 +2478,8 @@ if test "$want_winidn" = "yes"; then
  ]])
  ],[
  ],[
- 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"
+ AC_MSG_RESULT([no])
+ AC_MSG_ERROR([--with-winidn requires a minimum supported OS version of at least Vista])
  ])
  #
  CPPFLAGS="$CPPFLAGS $WINIDN_CPPFLAGS"