Makefile: let header dependency checker override COMPUTE_HEADER_DEPENDENCIES

Makefile: let header dependency checker override COMPUTE_HEADER_DEPENDENCIES

This way, if you have “COMPUTE_HEADER_DEPENDENCIES = YesPlease” in your
config.mak, you can still “make CHECK_HEADER_DEPENDENCIES=Yes” to check
the makefile after a successful build.

This change does not affect the result of the command
“make CHECK_HEADER_DEPENDENCIES=Yes COMPUTE_HEADER_DEPENDENCIES=Yes”.
That will still die with an error message:

	cannot compute header dependencies outside a normal build

The message is appropriate because still true.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>