LP#1334693 ./configure avoid osrf_config without core

Avoid the check for and use of osrf_config during Evergreen ./configure
when run with --disable-core.  This allows the staff client to be built
and, in general, for make_release to be run on a machine that does not
have opensrf installed (or has multiple versions).

Signed-off-by: Bill Erickson <berick@esilibrary.com>
Signed-off-by: Ben Shum <bshum@biblio.org>