fix build with json-c 0.12 if it provides a compatibility json.pc

json-c on openSUSE provides a compatibility pkg-config for "json".
The autoconf check find it first, assuming it does not provide
the function json_tokener_error_desc. Check for availability of
function using AC_CHECK_FUNCS instead to define the corresponding
HAVE_JSON_TOKENER_ERROR_DESC macro.
Merge pull request #125 from andreasstieger/json-c-0.12-autoconf

fix build with json-c 0.12 if it provides a compatibility json.pc