Incremental builds now take depfiles in account! Using a file that
contains a mapping of depfile to its cached contents, Zilch now
rebuilds less targets unnecessarily! If a build fails because an
include is added, it transparently rebuilds it with all possible header
dependencies, using the custom build scheduler built into Zilch. If an
include is removed, the target will be rebuilt with the new set of
headers the next time the CLI is invoked.
Change-Id: I6a6a6964c2fb191af4a474c45fd0f29623c588b0
