A makefile rule specifies a "depends-on" relationship between a target and some prerequisites.  I propose adding a new type of relationship to makefiles: an "includes" relationship.  Suppose, for the sake of example, this was written using "<" (less-than) instead of ":" (colon), the standard symbol for "depends-on" relationships.  You could write:

foo.c < foo.h bar.h

bar.h < tiger.h

foo.o : foo.c

This means foo.c includes the header files foo.h and bar.h, and the header file bar.h includes the header file tiger.h.  So foo.o has foo.c, foo.h, bar.h, and tiger.h as prerequisites.

The ability to specify "includes" relationships in this way would make it possible to simplify and speed up automated dependency generation.