How stupid of me. :>> doesn't change a modification time.

gnu/usr.bin/groff/contrib/mm/Makefile

@@ -26,7 +26,5 @@ beforeinstall:
 .if !exists(${DESTDIR}${TMACDIR}/mm/${locale})
 	${INSTALL} -o ${FILESOWN} -g ${FILESGRP} -m ${FILESMODE} \
 		/dev/null ${DESTDIR}${TMACDIR}/mm/${locale}
-.else
-	@:>>${DESTDIR}${TMACDIR}/mm/${locale}
 .endif
 .endfor

gnu/usr.bin/groff/tmac/Makefile

@@ -80,8 +80,6 @@ beforeinstall:
 .if !exists(${DESTDIR}${TMACDIR}/man.local)
 	${INSTALL} -o ${TMACOWN} -g ${TMACGRP} -m ${TMACMODE} \
 	    ${DIST_DIR}/man.local ${DESTDIR}${TMACDIR}
-.else
-	@:>>${DESTDIR}${TMACDIR}/man.local
 .endif
 
 .include <bsd.prog.mk>