https://git.suckless.org/dmenu/file/Makefile.html
https://git.suckless.org/sprop/file/Makefile.html
with file rule variables (my name for them, `OBJ = ${SRC:.c=.o}`).
https://github.com/lpsantil/rt0/blob/master/Makefile
https://github.com/lpsantil/oop0/blob/master/Makefile