GNU make doc - 3.8

Note that the directory prefix (D), as described in Implicit Rule Search Algorithm, is appended (after expansion) to all the patterns in the prerequisites list. As an example:

.SECONDEXPANSION:

/tmp/foo.o:

%.o: $$(addsuffix /%.c,foo bar) foo.h
        @echo $^

　　

The prerequisite list printed, after the secondary expansion and directory prefix reconstruction, will be /tmp/foo/foo.c /tmp/bar/foo.c foo.h. If you are not interested in this reconstruction, you can use $$* instead of % in the prerequisites list.