Problem appeared on mac and also on debian unstable. This is autotools
only as `--disable-since-check` is present
in meson.build.
Fix suggested by @nielsdg.
```
make[3]: Entering directory '/Users/distiller/.cache/jhbuild/build/gimp/extensions/goat-exercises'
CC goat-exercise-c.o
/Users/distiller/gtk/inst/bin/intltool-merge /Users/distiller/gtk/source/gimp/po-plug-ins org.gimp.extension.goat-exercises.metainfo.xml.in org.gimp.extension.goat-exercises.metainfo.xml -x -u -c ../../po-plug-ins/.intltool-merge-cache
VALAC goat_exercise_vala_vala.stamp
Generating and caching the translation database
CCLD goat-exercise-c
/Users/distiller/gtk/source/gimp/extensions/goat-exercises/goat-exercise-vala.vala:28.10-28.18: error: `Gimp.main' is not available in gimp-3.0 2.99.9. Use gimp-3.0 >= 3.0
return Gimp.main(typeof(Goat), args);
^^^^^^^^^
```
5ad5f67ad0