config: move packet mark support check to Bison file

src/config.c

@@ -545,10 +545,6 @@ void fastd_configure(int argc, char *const argv[]) {
 
 /** Performs some basic checks on the configuration */
 static void config_check_base(void) {
-#ifndef USE_PACKET_MARK
-	if (conf.packet_mark)
-		exit_error("config error: setting a packet mark is not supported on this system");
-#endif
 }
 
 /** Performs more checks on the configuration */

src/config.y

@@ -362,7 +362,12 @@ bind_default:
 	;
 
 packet_mark:	TOK_UINT {
+#ifdef USE_PACKET_MARK
 			conf.packet_mark = $1;
+#else
+			fastd_config_error(&@$, state, "setting a packet mark is not supported on this system");
+			YYERROR;
+#endif
 		}
 
 mtu:		TOK_UINT {

src/fastd.h

@@ -206,7 +206,9 @@ struct fastd_config {
 	uint16_t mtu;				/**< The configured MTU */
 	fastd_mode_t mode;			/**< The configured mode of operation */
 
+#ifdef USE_PACKET_MARK
 	uint32_t packet_mark;			/**< The configured packet mark (or 0) */
+#endif
 	bool forward;				/**< Specifies if packet forwarding is enable */
 	bool secure_handshakes;			/**< Can be set to false to support connections with fastd versions before v11 */