Fixed Makefile dependencies.

Fixed Makefile dependencies.

Support for some cases of "parallel make" was added on 2025/09/01, but poorly, because all object files now had a dependency on the object directory, which was modified whenever a new file was created, triggering recompilation of everything. This was fixed by singling out one object file (settings.o) whose compilation rule includes creation of build/obj (if necessary), and making all other build targets depend on settings.o.