configure.ac: Fix json-c detection for json-c >= 0.11

json-c-0.11 renamed the pkgconfig file to json-c
https://github.com/json-c/json-c/blob/master/ChangeLog The configure.ac
file was fixed to look for json-c if json is not available.

Signed-off-by: Markos Chandras <hwoarang@gentoo.org>
Signed-off-by: Martin Milata <mmilata@redhat.com>

libreport commit commit 07a33e562e99fa68332925c402538341d8cd82e2
Author: Markos Chandras <hwoarang@gentoo.org>

Signed-off-by: Jakub Filak <jfilak@redhat.com>