Makefile: add SANITIZE=leak flag to GIT-BUILD-OPTIONS

Makefile: add SANITIZE=leak flag to GIT-BUILD-OPTIONS

When SANITIZE=leak is specified we'll now add a SANITIZE_LEAK flag to
GIT-BUILD-OPTIONS, this can then be picked up by the test-lib.sh,
which sets a SANITIZE_LEAK prerequisite.

We can then skip specific tests that are known to fail under
SANITIZE=leak, add one such annotation to t0004-unwritable.sh, which
now passes under SANITIZE=leak.

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