kmk: .MUST_MAKE - hook for forcing a file to be made

[bird]

This is a new feature from the GNU Make TODO list that I've take a shot at because it will come in handy to deal with the $(commands ) mess/load.

The feature works like this, .MUST_MAKE is a target variable which will be looked for when a file is being considered for make but none of the prereqs requires it to be made. It is expanded in a limited file context, that means that $+, $^{, $? and $| are not readily available and weird stuff may yet happen because of VPATH. If .MUST_MAKE expands to a non-zero string after stripping, it will force the file to be made. Otherwise the file won't be made on its account at least.}

[bird]

Implemented in [2024].