Naturally, now that I've reported this bug after I couldn't find it being already reported, I search again and find that it was already reported a long time ago: <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=9587>.
- bug#67477: Automake warns that $(?D) is a non-POSIX variable n... Quinn Grier
- bug#67477: Automake warns that $(?D) is a non-POSIX varia... Karl Berry
- bug#67477: Automake warns that $(?D) is a non-POSIX varia... Quinn Grier