Makefile: improve SPARSE_FLAGS customisation
In order to enable greater user customisation of the SPARSE_FLAGS variable, we introduce a new SP_EXTRA_FLAGS variable to use for target specific settings. Without using the new variable, setting the SPARSE_FLAGS on the 'make' command-line would also override the value set by the target-specific rules in the Makefile (effectively making them useless). Also, this enables the SP_EXTRA_FLAGS to be used in the future for any other internal customisations, such as for some platform specific values. In addition, we initialise the SPARSE_FLAGS to the default (empty) value using a conditional assignment (?=). This allows SPARSE_FLAGS to be set from the environment as well as from the command-line. Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>