Makefile: help macOS novices by mentioning MacPorts

Makefile: help macOS novices by mentioning MacPorts

Since Aug 2006, the DarwinPorts project renamed themselves as
MacPorts.  Those who are not intimately familiar with the Opensource
ecosystem around macOS from olden days, the name DarwinPorts may not
ring a bell, even when they are using MacPorts.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>