Makefile: add a DEVOPTS to get all of -Wextra

Makefile: add a DEVOPTS to get all of -Wextra

Change DEVOPTS to understand a "extra-all" option. When the DEVELOPER
flag is enabled we turn on -Wextra, but manually switch some of the
warnings it turns on off.

This is because we have many existing occurrences of them in the code
base. This mode will stop the suppression, let the developer see and
decide whether to  fix them.

This change is a slight alteration of Nguyễn Thái Ngọc Duy
EAGER_DEVELOPER mode patch[1]

1. "[PATCH v3 3/3] Makefile: add EAGER_DEVELOPER
    mode" (<20180329150322.10722-4-pclouds@gmail.com>;
    https://public-inbox.org/git/20180329150322.10722-4-pclouds@gmail.com/)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>