Makefile.include: Replace PHONY by FORCE

[jcarrano]

Contribution description

Makefile.include abuses PHONY targets. These should be used for non-file targets, but they are currently being used to force re-running of the recipe.

This PR replaces PHONY targets by FORCE and clarifies when they should be used.

References

See the GNU Make manual:

• https://www.gnu.org/software/make/manual/html_node/Phony-Targets.html
• https://www.gnu.org/software/make/manual/html_node/Force-Targets.html

[cladmi]

I agree with the technical changes. I also checked that target that previously depended on $(RIOTBUILD_CONFIG_HEADER) depend on FORCE so will be rebuilt as before.

However, I do not really like the new documentation. See inline.

[cladmi]

The documentation does not unfortunately say that FORCE should be use for file targets. I already found that when adding FORCE in this file: #8844

Even if it was not that great, my previous documentation was mentioning the justification with the file timestamp being taken into account with FORCE contrary to .PHONY.
I find it important because it is not clear in the documentation and it explain why you should use FORCE.

[cladmi]

To be more precise on my thought, you changed to "how it should be used", I was more writing the "why it should be used", and unfortunately the official documentation does not say why. And I think both should be kept so I would prefer if you had a merge of both.

[jcarrano]

The make manual says: "A phony target should not be a prerequisite of a real target file". I take that to mean that if you make a target depend on a phony prerequisite, then that's equivalent to forcing the phony rule and the target's rule to be run anytime. This is what we are doing by defining FORCE as phony and making everything depend on force.

If the file is updated each time the rule runs, it should not change anything. What FORCE gives us is the possibility of choosing not to update the file and then it will not trigger an update in it's dependents.

Some other considerations: you cannot make a pattern rule phony, but you can make it FORCE.

Should I clarify all that in the comment?

[cladmi]

If the file is updated each time the rule runs, it should not change anything. What FORCE gives us is the possibility of choosing not to update the file and then it will not trigger an update in it's dependents.

That what I explained with the previous documentation. So just keep it I guess and maybe clarify it.

No need to explain everything, keep it simple. Adding your part with "file targets -> FORCE" and "non file -> .PHONY" would be enough.

[jcarrano]

Some other considerations: you cannot make a pattern rule phony, but you can make it FORCE.

I correct myself, what the manual says is "The implicit rule search (see Implicit Rules) is skipped for .PHONY targets."

[cladmi]

Nitpick, replace PHONY by .PHONY but otherwise I like it this way.

[cladmi]

ACK please squash the documentation commits

[jcarrano]

Squashed and rebased.