How does the C# compiler decide to emit retargetable assembly references?

Question

Retargetable assembly references have been introduced for the .NET Compact Framework and are now used to support Portable Class Libraries.

Basically, the compiler emits

Answer 1

I've noticed by experimenting that the C# compiler would make an reference compiler as retargetable if the referenced assembly is marked as retargetable (a modifier on the .assembly section in MSIL). I did not find how the compiler decides to make the assembly retargetable, yet.