Simple routines to speed up build by only rebuilding packages whose source (or in-tree dependencies) have changed.
Since our build involves a lot of separate invocations of build tools on the same (symlinked) directories over and over, it's a bit wasteful to keep on calculating the same checksums.
Speed up the build by setting the environment variable:
MERKLE_BUILD_CACHE
To point to a temporary directory. If set, that directory will be used
as an out-of-process cache for the merkle-build library.
Since cache invalidation is one of the hard problems of computer science, you should be careful to not introduce inconsistencies by (accidentally) sharing the cache between build runs.