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>