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>