Makefile: minor improvements for Mac OS X (Darwin)

Makefile: minor improvements for Mac OS X (Darwin)

1) Instead of requesting OLD_ICONV on all Mac OS X versions except for 10.5
(which will break when 10.6 is released), exlicitly request it for versions
older than 10.5.

2) NO_STRLCPY is not needed since Mac OS X 10.2. Noticed by Benjamin Kramer.

Note that uname -r returns the underlying Darwin version, which can be mapped
to Mac OS X version at http://www.opensource.apple.com/darwinsource/

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>