NB: I found on some (very old) Roxen docs that "Informix is supported via native driver", but I couldn't find any traces of this driver anywhere... Perhaps this was a mistake... Or joke :)
The above statement refers to the Odbc driver. INFORMIX-CLI at the time (~1996) was just another name for ODBC (cf "INFORMIX-CLI Programmer's Manual Version 2.5", Informix Press, August 1996, Part No. 000-8984).
Regards, /Al