I am aware of Package Manager Extensible Build Tools however I am on certain schedule since the use case is open source project I want to/have to publish before June so even if such a feature would be in review now, it won't be in swiftpm soon enough for me.
I'm not saying I like such an approach, but I think, that swift build should be the only thing you write into terminal in order to build a package. I dislike packages with "install.sh" files more than I dislike some magic inside Package.swift file. At the same time, Xcode is not an issue for me, since I aim at Linux platform.
In case external build systems would be supported directly by SwiftPM I would be more than happy to get advantage of such a feature.
I was wondering, however, whether such an approach is dangerous. As @lukasa mentioned with Package.resolved or outright forbidden.
Thanks for your opinions so far :)