Yet another synonym for ASCII: "646"

Used by NetBSD; this unbreaks mprintf() and friends there for the C
locale (caught by dtucker@ and his menagerie of test systems).
diff --git a/utf8.c b/utf8.c
index f2c89a2..d389020 100644
--- a/utf8.c
+++ b/utf8.c
@@ -61,7 +61,7 @@
 
 	loc = nl_langinfo(CODESET);
 	return strcmp(loc, "US-ASCII") != 0 && strcmp(loc, "UTF-8") != 0 &&
-	    strcmp(loc, "ANSI_X3.4-1968") != 0;
+	    strcmp(loc, "ANSI_X3.4-1968") != 0 strcmp(loc, "646") != 0;
 }
 
 static int