Makefile: Use computed header dependencies if the compiler supports it

Makefile: Use computed header dependencies if the compiler supports it

Previously you had to manually define COMPUTE_HEADER_DEPENDENCIES to
enable this feature. It seemed a bit sad that such a useful feature
had to be enabled manually.

To avoid the small overhead we don't do the auto-detection if
COMPUTE_HEADER_DEPENDENCIES is already set.

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