machinekit-cnc is a split out repo which just contains the CNC elements of machinekit
The main reasons for stripping out the CNC stack, are
Installing packages machinekit-hal-{flavour} followed by machinekit-cnc-{flavour} will give you a fully operational version of machinekit.
If you want to build as a RIP, you need to clone machinekit-hal.
From the root of that clone run scripts/build_with_cnc
This will clone machinekit-cnc into the root of the machinekit-hal clone.
machinekit-hal is built, then machinekit-cnc is built with necessary artifacts from machinekit-hal build copied over.
Then the required libs/binaries/scripts/configs etc are copied into machinekit-hal and the environment var is set ready to run the RIP