Make dependency on Mana mod optional

depends.txt

@@ -1 +1 @@
-mana
+mana?

init.lua

@@ -1,6 +1,12 @@
 returnmirror = {}
 returnmirror.cost = 100
 
+if minetest.get_modpath("mana") ~= nil then
+	returnmirror.mana = true
+else
+	returnmirror.mana = false
+end
+
 minetest.register_tool("returnmirror:returnmirror", {
 	description = "Mirror of Returning",
 	stack_max = 1,
@@ -11,7 +17,17 @@ minetest.register_tool("returnmirror:returnmirror", {
 		local dest_string = itemstack:get_metadata()
 		local dest = minetest.string_to_pos(dest_string)
 		if dest ~= nil then
-			if mana.subtract(user:get_player_name(), returnmirror.cost) then
+			local allowed
+			if returnmirror.mana then
+				if mana.subtract(user:get_player_name(), returnmirror.cost) then
+					allowed = true
+				else
+					allowed = false
+				end
+			else
+				allowed = true
+			end
+			if allowed then
 				local src = user:getpos()
 				minetest.sound_play( {name="returnmirror_teleport", gain=1}, {pos=src, max_hear_distance=30})
 				minetest.add_particlespawner({