Make pg_probackup works again on PostgreSQL 9.5

Function pg_control_system is added in 9.6, which makes check on
9.5 fails all the time. Added logic to bypass that checking of
system identifier between data directory DB connection unless
DB version is >= 9.6

Author: rtau-t24

src/backup.c

@@ -934,22 +934,24 @@ check_system_identifiers(void)
 
 	system_id_pgdata = get_system_identifier(pgdata);
 
-	res = pgut_execute(backup_conn,
+	if (server_version < 90600) {
+	} else {
+		res = pgut_execute(backup_conn,
 					   "SELECT system_identifier FROM pg_control_system()",
 					   0, NULL);
-	val = PQgetvalue(res, 0, 0);
+		val = PQgetvalue(res, 0, 0);
 
-	if (!parse_uint64(val, &system_id_conn))
-	{
+		if (!parse_uint64(val, &system_id_conn))
+		{
+			PQclear(res);
+			elog(ERROR, "%s is not system_identifier", val);
+		}
 		PQclear(res);
-		elog(ERROR, "%s is not system_identifier", val);
-	}
-	PQclear(res);
-
-	if (system_id_conn != system_identifier)
-		elog(ERROR, "Backup data directory was initialized for system id %ld, but connected instance system id is %ld",
-			 system_identifier, system_id_conn);
 
+		if (system_id_conn != system_identifier)
+			elog(ERROR, "Backup data directory was initialized for system id %ld, but connected instance system id is %ld",
+				 system_identifier, system_id_conn);
+	}
 	if (system_id_pgdata != system_identifier)
 		elog(ERROR, "Backup data directory was initialized for system id %ld, but target backup directory system id is %ld",
 			 system_identifier, system_id_pgdata);