Add the LAST_ARG_MUST_BE_NULL macro

The sentinel function attribute is not understood by versions of
the gcc compiler prior to v4.0. At present, for earlier versions
of gcc, the build issues 108 warnings related to the unknown
attribute. In order to suppress the warnings, we conditionally
define the LAST_ARG_MUST_BE_NULL macro to provide the sentinel attribute
for gcc v4.0 and newer.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>