bugfix: error: json_tokener_errors undeclared when overriding PKGCONFIG

If PKGCONFIG settings for json-c were overriden, presence of
json_tokener_errors was not properly detected.
closes https://github.com/rsyslog/rsyslog/issues/143
maintain ChangeLog
Merge branch 'v7-stable' into v8-stable
Merge branch 'v8-stable'