Define NO_MEMMEM on Darwin as it lacks the function

Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -398,6 +398,7 @@ ifeq ($(uname_S),Darwin)
 	NEEDS_LIBICONV = YesPlease
 	OLD_ICONV = UnfortunatelyYes
 	NO_STRLCPY = YesPlease
+	NO_MEMMEM = YesPlease
 endif
 ifeq ($(uname_S),SunOS)
 	NEEDS_SOCKET = YesPlease