Similar to the build epoch check, cache a list of source options in the
object tree, and force a clean build if the cached list does not match
the current list, after filtering out options which are known not to
affect the build (e.g. CLEAN, TESTS, WARNS).
This also adds a DEPEND_CLEANUP option (which defaults to yes unless
the CLEAN option is set) which can be used to skip depend-cleanup for
faster incremental builds.