Makefile: OS X has /dev/tty

We can use our enhanced getpass(). Tested by me.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -898,6 +898,7 @@ ifeq ($(uname_S),Darwin)
 	endif
 	NO_MEMMEM = YesPlease
 	USE_ST_TIMESPEC = YesPlease
+	HAVE_DEV_TTY = YesPlease
 endif
 ifeq ($(uname_S),SunOS)
 	NEEDS_SOCKET = YesPlease