Teach Makefile to check header dependencies

Teach Makefile to check header dependencies

Add a target to use the gcc-generated makefile snippets for
dependencies on header files to check the hard-coded dependencies.

With this patch applied, if any dependencies are missing, then

	make clean
	make COMPUTE_HEADER_DEPENDENCIES=YesPlease
	make CHECK_HEADER_DEPENDENCIES=YesPlease

will produce an error message like the following:

	CHECK fast-import.o
	missing dependencies: exec_cmd.h
	make: *** [fast-import.o] Error 1

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>