This macro will generate a #warning message when we'll move on to 3.99
series (which means we will be on our way to GIMP 4). And it will become
an #error message on 4.0.0, hence preventing us from releasing unless we
actually resolve all these warnings.
Resolution may mean actually breaking the API/ABI, or just deciding that
it's not a good idea in the end, or finding another solution. But
something will have to be decided.
Please everyone use this macro when you discover issues where it looks
like we could improve the API (in a breaking way) so that we don't
forget when approaching GIMP 4 (pushing further the improvement).
Updating the 2 place where I was already using a GIMP_CHECK_VERSION()
with #warning, and adding a new usage in the compose plug-in, per
discussion in MR !2424.
b1e52381f5