Simply have an [x86_64-elf-gcc](https://github.com/lordmilko/i686-elf-tools) and ld (included!) in your PATH, run `cmake` in the source directory, then `make`.
It will compile the kernel, and create an OS image with `mkbootimg`.
The system for linux is a lot easier, but you *do* need an x86_64-elf-gcc cross compiler. You can get one from the AUR on Arch-based distros (like Manjaro), or make one yourself using [the OSDev Wiki guide](https://wiki.osdev.org/GCC_Cross-Compiler)