Makefile: add a DEVOPTS flag to get pedantic compilation

Makefile: add a DEVOPTS flag to get pedantic compilation

In the interest of code hygiene, make it easier to compile Git with the
flag -pedantic.

Pure pedantic compilation with GCC 7.3 results in one warning per use of
the translation macro `N_`:

    warning: array initialized from parenthesized string constant [-Wpedantic]

Therefore also disable the parenthesising of i18n strings with
-DUSE_PARENS_AROUND_GETTEXT_N=0.

Signed-off-by: Beat Bolli <dev+git@drbeat.li>
Signed-off-by: Junio C Hamano <gitster@pobox.com>