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

abertelrud/swift-package-manager,sschiau/swift-package-manager,haskellswift/swift-package-manager,sschiau/swift-package-manager,mxcl/swift-package-manager,haskellswift/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,vmanot/swift-package-manager,sschiau/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,mxcl/swift-package-manager,vmanot/swift-package-manager,apple/swift-package-manager,sschiau/swift-package-manager,mxcl/swift-package-manager,sschiau/swift-package-manager,mxcl/swift-package-manager,vmanot/swift-package-manager,vmanot/swift-package-manager,apple/swift-package-manager,haskellswift/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,haskellswift/swift-package-manager,vmanot/swift-package-manager,apple/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager