Silence iconv warnings on Leopard

Silence iconv warnings on Leopard

Apple ships a newer version of iconv with Leopard (Mac OS X 10.5/Darwin
9). Ensure that OLD_ICONV is not set on any version of Darwin in the
9.x series; this should be good for at least a couple of years, when
Darwin 10 comes out and we can invert the sense of the test to
specifically check for Darwin 7 or 8.

A more sophisticated and robust check is possible for those who use
autoconf, but not everybody does that.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>