When a dependency is found in another directory through directory search,
this cannot change the commands of the rule; they will execute as written.
Therefore, you must write the commands with care so that they will look for
the dependency in the directory where make finds it.
This is done with the automatic variables such as `$^' (see section Automatic Variables). For instance, the value of `$^' is a list of all the dependencies of the rule, including the names of the directories in which they were found, and the value of `$@' is the target. Thus:
foo.o : foo.c
cc -c $(CFLAGS) $^ -o $@
(The variable CFLAGS exists so you can specify flags for C
compilation by implicit rules; we use it here for consistency so it will
affect all C compilations uniformly;
see section Variables Used by Implicit Rules.)
Often the dependencies include header files as well, which you do not want to mention in the commands. The automatic variable `$<' is just the first dependency:
VPATH = src:../headers
foo.o : foo.c defs.h hack.h
cc -c $(CFLAGS) $< -o $@