Mix: New option vars_config_path to set path to vars.config (4128)

Useful when setting ejabberd as dependency in another Elixir project,
and you want to enable or disable some ejabberd dependencies, see
https://www.process-one.net/blog/how-to-use-ejabberd-as-an-elixir-application-dependency/

mix.exs

@@ -171,7 +171,13 @@ defmodule Ejabberd.MixProject do
   end
 
   defp vars do
-    case :file.consult("vars.config") do
+    filepath = case Application.fetch_env(:ejabberd, :vars_config_path) do
+      :error ->
+        "vars.config"
+      {:ok, path} ->
+        path
+    end
+    case :file.consult(filepath) do
       {:ok,config} -> config
       _ -> [zlib: true]
     end