Allow json-c lib named either "json" or "json-c"

With version 0.11, the json-c library (https://github.com/json-c/json-c/wiki)
renamed their pkgconfig file from "json" to "json-c".

This way, either name is accepted and the website hinting on failure is
preserved.

See also: https://bugs.gentoo.org/show_bug.cgi?id=467354
Merge pull request #77 from eroen/allow-json-c-lib-named-either

Allow json-c lib named either "json" or "json-c"