Use --package-path instead of the deprecated -C flag.

[kaandedeoglu]

This fixes a build issue when using Marathon with Swift 4.

This PR should probably wait here until Xcode 9 & Swift 4 is officially out.

[JohnSundell]

Thanks for this @kaandedeoglu! 🙌 How about doing this only if the user is running Swift 4? I don't want to be too aggressive dropping support for Swift 3, since people might have scripts running on CI etc that might take a while to get on Swift 4/Xcode 9.

PackageManager has a resolveSwiftToolsVersion() method which can be used to resolve the current Swift version. It's private right now, but we could move that somewhere where Script can access it as well.

[kaandedeoglu]

Sure thing! I'll take a look and restrict the change for Swift 4+. And not a problem at all, I love Marathon :-)

[JohnSundell]

@kaandedeoglu awesome to hear 🎉

[dyna-dot]

Hi