adding *.h dependency, problem

[Yakov Lerner]

I have tiny makefile that looks like this:

all: prog
prog: a.o b.o ; $(CC) -o $@ a.o b.o

It is very short because it relies on builtin %.o: %.c rule.
How do I add the following dependency:  that all *.c files depend on all *.h ?
So that when any *.h changes, all *.o are recompiled ?
I tried to add '%.o: %.c *.h' now, but it didn't do the trick.

Thanks
Yakov