To successfully call the `capnp_generate_cpp()` function, the
`libmultiprocess` build system must be provided with paths to the native
`capnp` and `capnpc-c++` tools.
When no build dir is specified, cmake will warn:
```bash
Preprocessing libmultiprocess...
Configuring libmultiprocess...
CMake Warning:
No source or binary directory provided. Both will be assumed to be the
same as the current working directory, but note that this warning will
become a fatal error in future CMake releases.
```
It's unclear if this will actually ever become an error, but it's also easy
enough to just supply the directory, and save this maybe breaking in
future.
