Don't try to load NIF if not built

If building the NIF was disabled, don't even try to load it. The NIF
functions will return errors, but that will hopefully be unsurprising.

Author: fhunleth

lib/i2c/i2c_nif.ex

@@ -6,11 +6,13 @@
 defmodule Circuits.I2C.Nif do
   @moduledoc false
 
-  @on_load {:load_nif, 0}
-  @compile {:autoload, false}
+  if System.get_env("CIRCUITS_I2C_I2CDEV") != "disabled" do
+    @on_load {:load_nif, 0}
+    @compile {:autoload, false}
 
-  def load_nif() do
-    :erlang.load_nif(:code.priv_dir(:circuits_i2c) ++ ~c"/i2c_nif", 0)
+    def load_nif() do
+      :erlang.load_nif(:code.priv_dir(:circuits_i2c) ++ ~c"/i2c_nif", 0)
+    end
   end
 
   def open(_device, _timeout), do: :erlang.nif_error(:nif_not_loaded)