lib: prefix macros UNUSED and FALLTHROUGH

[nilason]

Prefix the preprocessing macros UNUSED and FALLTHROUGH with G_ to avoid conflicts with third party software.

This is something I have long planned to do. For example, there was an issue for QGIS, which they solved in qgis/QGIS#55018; one of the addons was also affected OSGeo/grass-addons#1626.

[echoix]

Read fast on all files, looks fine.
How will we communicate the changes?
Is there something to adapt for the ctypegen python bindings?

What bugs me a bit is that we can't really rely on the Doxygen docs to show the change, as it is only one version for all grass8, and it kinda changes a lot. Like, we wouldn't know for grass8.4 what was possible?

[nilason]

Read fast on all files, looks fine. How will we communicate the changes?

It is supposed to be of internal use, so I think not necessary.

Is there something to adapt for the ctypegen python bindings?

No, this is for c/c++ compiler only.

What bugs me a bit is that we can't really rely on the Doxygen docs to show the change, as it is only one version for all grass8, and it kinda changes a lot. Like, we wouldn't know for grass8.4 what was possible?

Thanks, doxygen needs to be updated, just pushed a commit! Otherwise the macro is only to silence compiler warnings.