Makefile: use curl-config to determine curl flags

Makefile: use curl-config to determine curl flags

curl-config should always be installed alongside a curl distribution,
and its purpose is to provide flags for building against libcurl, so
use it instead of guessing flags and dependent libraries.

Allow overriding CURL_CONFIG to a custom path to curl-config, to
compile against a curl installation other than the first in PATH.

Depending on the set of features curl is compiled with, there may be
more libraries required than the previous two options of -lssl and
-lidn. For example, with a vanilla build of libcurl-7.36.0 on Mac OS X
10.9:

$ ~/d/curl-out-7.36.0/lib/curl-config --libs
-L/Users/dborowitz/d/curl-out-7.36.0/lib -lcurl -lgssapi_krb5 -lresolv -lldap -lz

Use this only when CURLDIR is not explicitly specified, to continue
supporting older builds.

Signed-off-by: Dave Borowitz <dborowitz@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>