upgrade the default WINNT version

* configure.ac: set the default target Windows NT version to 0x0600,
  as well as mswin version since r50051.  Windows XP has ended years
  ago.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64294 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.ac

@@ -687,8 +687,8 @@ test -z "${ac_env_CXXFLAGS_set}" -a -n "${cxxflags+set}" && eval CXXFLAGS="\"$cx
 
 : "header and library section" && {
 AC_ARG_WITH(winnt-ver,
-  AS_HELP_STRING([--with-winnt-ver=0xXXXX], [target Windows NT version (default to 0x0501)]),
-  [with_winnt_ver="$withval"], [with_winnt_ver="0x0501"])
+  AS_HELP_STRING([--with-winnt-ver=0xXXXX], [target Windows NT version (default to 0x0600)]),
+  [with_winnt_ver="$withval"], [with_winnt_ver="0x0600"])
 AS_CASE(["$target_os"],
 [mingw*], [
   RUBY_APPEND_OPTION(CPPFLAGS, -D_WIN32_WINNT=$with_winnt_ver)