default: get darkage modpath to ensure basalt is available

mods/default/functions.lua

@@ -101,10 +101,14 @@ end
 --
 
 lavagen = {
-"darkage:basalt",
 "default:stone"
 }
 
+-- thanks to ElectronLibre from Minetest IRC
+if minetest.get_modpath("darkage")
+    then table.insert(lavagen, "darkage:basalt")
+end
+
 default.cool_lava = function(pos, node)
 	if node.name == "default:lava_source" then
 		minetest.set_node(pos, {name = "default:obsidian"})