Makefile: add $(DEVELOPER_CFLAGS) variable
This does not change the behavior, but allows the user to tweak
DEVELOPER_CFLAGS on the command-line or in a config.mak* file if
needed.
This also makes the code somewhat cleaner as it follows the pattern
<initialisation of variables>
<include statements>
<actual build logic>
by specifying which flags to activate in the first part, and actually
activating them in the last one.
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>