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>