Errors with Extension methods after update.

Hi,

I have just updated Xamarin Studio to latest stable release, but now I cannot build the project that was working just fine before the update. I am running OSX 10.9 with Xamarin Studio (4.2.2 (build 2)), Xamarin.Android (4.10.1) and Mono 3.2.5 ((no/964e8f0).

I get errors regarding the extension attributes: Extension methods require `System.Runtime.CompilerServices.ExtensionAttribute' type to be available. Are you missing an assembly reference?

but I am sure that all the assemblies should be there. If you would like to test here is the my fork of MonoGame that is causing the errors: https://github.com/thes3m/MonoGame

Thanks