swift-package-manager

[Xcodeproj] Use SUPPORTED_PLATFORMS to allow generated targets to build for any platform.

 - The model we are trying to support is one where the generated project itself
   supplies targets which *can be* built for any platform, and to then drive
   exactly what targets it *is* built for by the project which integrates this
   project (in the case where SwiftPM is being used to build dependencies of an
   existing application).