Workaround/fixes for cJSON symbol collision

Provide 2 new confiration options "disable_json" and "external_cjson" external_cjson is intended ONLY for static libraries that get linked to an application that already has cjson compiled in, so that it may use that. The version of cJSON MUST match the version used in libRIST or at least be ABI compatible. Would fix #111 (closed)