build: specify cmake build dir for multiprocess depends build

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.

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