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
this does not change the API exposed by the library, so it'll be safe
for users to substitute stock capnp. we do want it for our own builds
though, at least until we can switch to capnp 2 or fully go for rust.
Change-Id: Ia8ac6af0165f6f61ee02345179dc0a93a0889a06