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>