Makefile: iconv() on Darwin has the old interface

The libiconv on Darwin uses the old iconv() interface (2nd argument is a
const char **, instead of a char **).  Add OLD_ICONV to the Darwin
variable definitions to handle this.
Signed-off-by: NArjen Laarhoven <arjen@yaph.org>
Acked-by: NBrian Gernhardt <benji@silverinsanity.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Makefile

@@ -356,6 +356,7 @@ endif
 ifeq ($(uname_S),Darwin)
 	NEEDS_SSL_WITH_CRYPTO = YesPlease
 	NEEDS_LIBICONV = YesPlease
+	OLD_ICONV = UnfortunatelyYes
 	NO_STRLCPY = YesPlease
 endif
 ifeq ($(uname_S),SunOS)