Roger's suggestion doesn't actually do it in the mkfile, though I expect it
could. This is what I do....

COMMON=/somewhereelse

%.$O:	$COMMON/%.c
	$O^c $CFLAGS $COMMON/$stem.c

thingy: local.$O common.$O
	$O^c -o thingy local.$O common.$O

or words to that effect.