When ENABLE_IPC option is on, build with libmultiprocess subtree and
`add_subdirectory(src/ipc/libmultiprocess)` instead of external package
and `find_package(Libmultiprocess)` by default.
Behavior can be toggled with `WITH_EXTERNAL_LIBMULTIPROCESS` option. Using a
subtree should be more convenient for most bitcoin developers, but using an
external package is more convenient for developing in the libmultiprocess
repository.
The `WITH_EXTERNAL_LIBMULTIPROCESS` option is also used to avoid needing to
changing the depends build here. But in later commits, the depends build is
switched to use the add_subdirectory build as well.
Co-authored-by: Cory Fields <cory-nospam-@coryfields.com>
Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>
