Enable building rivers in-game.

Controlled by setting, default off.

src/settings_type.h

@@ -311,6 +311,7 @@ struct ConstructionSettings {
 	bool   freeform_edges;                   ///< allow terraforming the tiles at the map edges
 	uint8  extra_tree_placement;             ///< (dis)allow building extra trees in-game
 	uint8  command_pause_level;              ///< level/amount of commands that can't be executed while paused
+	bool   enable_build_river;               ///< enable building rivers in-game
 
 	uint32 terraform_per_64k_frames;         ///< how many tile heights may, over a long period, be terraformed per 65536 frames?
 	uint16 terraform_frame_burst;            ///< how many tile heights may, over a short period, be terraformed?