Fix some warnings from mix

lib/geef/object.ex

@@ -44,10 +44,10 @@ defmodule Geef.Object do
     Tree.get(tree, key)
   end
 
-  def get(tree = %Object{type: :tree}, key) do
+  def get(tree = %Object{type: :tree}, key, default \\ nil) do
     case fetch(tree, key) do
       {:ok, entry} -> entry
-      {:error, _} -> nil
+      {:error, _} -> default
     end
   end
 
@@ -56,4 +56,8 @@ defmodule Geef.Object do
     raise ArgumentError
   end
 
+  def pop(_tree, _key) do
+    raise ArgumentError
+  end
+
 end

mix.exs

@@ -54,10 +54,12 @@ defmodule Mix.Tasks.Compile.Nif do
     exts = Keyword.get(config, :exts, [:c])
     compiler = Keyword.get(config, :compilers, ["cc", "gcc", "clang"]) |> find_compiler
 
-    if cflags = System.get_env("CFLAGS") do
+    flags =
+      case cflags = System.get_env("CFLAGS") do
 	# FIXME: this isn't going to work too well with quoted spaces
 	# inside arguments
-      flags = [flags, String.split(cflags)]
+	true -> [flags, String.split(cflags)]
+	_ -> flags
       end
 
     # Create the directory (e.g. "priv/")