This partially reverts commit f68233ec43.
while more modern capnp in *does* have the build system changes, it
does *not* have the patches. and those patches are rather important
to us; the clock patch because our CI machiens don't behave the way
capnp expect and the nodiscard patch because it *is* a bugsquasher.
Change-Id: Ic273777f09fd8e5e7001f815db8998dae2dd7b88