Make OpenVMS get auto-detected. r=leaf

build/autoconf/config.guess

@@ -975,6 +975,14 @@ EOF
                            # says <Richard.M.Bartel@ccMail.Census.GOV>
         echo i586-unisys-sysv4
         exit 0 ;;
+    *:*OpenVMS*:*:*)
+	case "${UNAME_SYSTEM}" in
+	    POSIX_for_OpenVMS_AXP) echo alpha-dec-openvmsposix ;;
+	    POSIX_for_OpenVMS_VAX) echo vax-dec-openvmsposix ;;
+	    OpenVMS) echo alpha-dec-openvms ;;
+	    *) echo unknown-dec-openvms ;;
+	esac
+	exit 0 ;;
     *:UNIX_System_V:4*:FTX*)
 	# From Gerald Hewes <hewes@openmarket.com>.
 	# How about differentiating between stratus architectures? -djm