Arduino printf/fprintf/sprintf prints question mark instead of float

Using embedded coder to build an ArduinoMEGA2560 target. I need to do "sprintf" manipulations. I used "custom sfunction builder" to add sprintf. That works great for formatting integer arguments but I get "?" when I attempt to use "%f" format.

Found this general issue "question mark instead of float" discussed on StackOverflow: http://stackoverflow.com/questions/14146850/arduino-printf-fprintf-prints-question-mark-instead-of-float

Problem can be solved by passing the -lprintf_flt option to the avr linker.

Does anyone know how to add linker flags to an Arduino embedded coder target?