Add OS X-specific SharedEnvExtension.

Library/Homebrew/extend/ENV/shared.rb

@@ -329,3 +329,5 @@ module SharedEnvExtension
     version && Version.create(version) >= Version.create("4.8")
   end
 end
+
+require "extend/os/extend/ENV/shared"

Library/Homebrew/extend/os/extend/ENV/shared.rb

+require "extend/ENV/shared"
+
+if OS.mac?
+  require "extend/os/mac/extend/ENV/shared"
+end

Library/Homebrew/extend/os/mac/extend/ENV/shared.rb

+module SharedEnvExtension
+end