Issue #9642: Uniformize the tests on the availability of the mbcs codec

Add a new HAVE_MBCS define.
diff --git a/Misc/NEWS b/Misc/NEWS
index 350427e..6678ace 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@
 Core and Builtins
 -----------------
 
+- Issue #9642: Uniformize the tests on the availability of the mbcs codec, add
+  a new HAVE_MBCS define.
+
 - Issue #9642: Fix filesystem encoding initialization: use the ANSI code page
   on Windows if the mbcs codec is not available, and fail with a fatal error if
   we cannot get the locale encoding (if nl_langinfo(CODESET) is not available)