Enable building rivers in-game.

Controlled by setting, default off.

src/table/settings.ini

@@ -418,6 +418,15 @@ str      = STR_CONFIG_SETTING_COMMAND_PAUSE_LEVEL
 strhelp  = STR_CONFIG_SETTING_COMMAND_PAUSE_LEVEL_HELPTEXT
 strval   = STR_CONFIG_SETTING_COMMAND_PAUSE_LEVEL_NO_ACTIONS
 
+[SDT_BOOL]
+base     = GameSettings
+var      = construction.enable_build_river
+def      = false
+cat      = SC_BASIC
+str      = STR_CONFIG_SETTING_ENABLE_BUILD_RIVER
+strhelp  = STR_CONFIG_SETTING_ENABLE_BUILD_RIVER_HELPTEXT
+patxname = ""enable_build_river.construction.enable_build_river""
+
 [SDT_VAR]
 base     = GameSettings
 var      = construction.terraform_per_64k_frames