locale.c: langinfo: Use Windows fcn to find CODESET

There is a Windows function, available for quite a long time, that will
return the current code page.  Use this for the nl_langinfo() CODESET,
as that libc function isn't implemented on Windows.

locale.c

@@ -3525,8 +3525,21 @@ S_my_langinfo(pTHX_
 
 #  else
 
-        {   /* But on Windows, the name does seem to be consistent, so
-               use that. */
+        {
+            /* This function retrieves the code page.  It is subject to change,
+             * but is documented and has been stable for many releases */
+            UINT ___lc_codepage_func(void);
+
+            retval = save_to_buffer(Perl_form(aTHX_ "%d", ___lc_codepage_func()),
+                                    retbufp, retbuf_sizep);
+            DEBUG_Lv(PerlIO_printf(Perl_debug_log, "locale='%s' cp=%s\n",
+                                                   locale, retval));
+            return retval;
+        }
+
+#  endif
+
+        {   /* Temporarily unreachable */
             const char * name = querylocale_c(LC_CTYPE);
 
             if (isNAME_C_OR_POSIX(name)) {