From `configure --help`:
--with-external-capnp use the system capnp binary (or the one specified
with $CAPNP) instead of compiling a new one (useful
for cross-compiling)
The non-native capnp was trying to fetch the wrong file.
Without this, "make -C depends MULTIPROCESS=1 download" is broken.
Presumably it breaks with the download target because the dependency graph is
flattened. It manages to work if native_capnp is encountered first because it
will then be found in the cache.
