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.

> checking for JSON_C... yes
> checking for json_object_new_object in -ljson-c... yes
> checking for json_object_new_int64... yes
> checking for json_tokener_error_desc... yes

See rsyslog@6b47dd542d07ed557f02af9970d1004eb262e1c1