Changes since 23.13.0:
Fix wrong warnings about MASTER_SITE_BACKUP being not known.
Reduce the amount of log data when tracing variables that repeatedly use += to append to a variable.
Support the .-include directive, the .ifmake directive and the built-in < variable, to be able to check standalone makefiles outside pkgsrc.
Complain about merge conflicts in makefiles, which were silently ignored before.
Complain about package patterns whose bounds contradict each other, such as pkgbase>=2<1, as these never match and are probably typos.
Do not warn about pkgsrc-wip packages that are missing a COMMIT_MSG file, as long as they have a TODO file, as it is fine to have work-in-progress packages in pkgsrc-wip.