[$] Merkle trees and build systems

In traditional build tools like Make, targets and dependencies are alwaysfiles. Imagine if you could specify an entire tree (directory) as adependency: You could exhaustively specify a "build root" filesystem containingthe toolchain used for building some target as a dependency of that target.Similarly, a rule that creates that build root would have the tree as itstarget.Using Merkletrees as first-class citizens in a build system gives greatflexibility and many optimization opportunities. In this article, guest author David Rothlisbergerexplores this idea using OSTree,Ninja, and Python.