Disable make·file prerequisites when not needed

Disable make·file prerequisites when not needed

Specifically, the `help` and `clean` targets don’t require any
compilation of types, parsers, dependencies, or destinations. G·N·U
Make provides the `$(MAKECMDGOALS)` to check which targets were set as
goals on the commandline; when the only goals are `help` and `clean`,
excessive computation can be disabled.