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