osxkeychain: pull make config from top-level directory

osxkeychain: pull make config from top-level directory

The default compiler and cflags were mostly "works for me"
when I built the original version. We need to be much less
careful here than usual, because we know we are building
only on OS X.  But it's only polite to at least respect the
CFLAGS and CC definitions that the user may have provided
earlier.

While we're at it, let's update our definitions and rules to
be more like the top-level Makefile; default our CFLAGS to
include -O2, and make sure we use CFLAGS and LDFLAGS when
linking.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>