Is it possible to change the build settings in Xamarin Studio 5.5.3 so that some errors are interpreted as warnings? E.g. a missing reference is no build error anymore?
How can I do this?
A missing reference is always a build error. In the case of Xamarin, things like linking, AOT, compile to native, etc. can't be performed at all to create an application package.
You should include the missing DLL in your repository, so it is always there.
Or better yet, distribute it via NuGet so you can update it. You can host your own NuGet feed for this purpose or try MyGet.