config.mak.uname: Darwin: define HAVE_GETDELIM for modern OS X releases

config.mak.uname: Darwin: define HAVE_GETDELIM for modern OS X releases

On Mac OS X, getdelim() first became available with Xcode 4.1[1], which
was released the same day as OS X 10.7 "Lion", so assume getdelim()
availability from 10.7 onward. (As of this writing, OS X is at 10.10
"Yosemite".)

According to Wikipedia[2], 4.1 was also available for download by paying
developers on OS X 10.6 "Snow Leopard", so it's possible that some 10.6
machines may have getdelim(). However, as strbuf's use of getdelim() is
purely an optimization, let's be conservative and assume 10.6 and
earlier lack getdelim().

[1]: Or, possibly with Xcode 4.0, but that version is no longer
     available for download, or not available to non-paying developers,
     so testing is not possible.

[2]: http://en.wikipedia.org/wiki/Xcode

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>