This is the has-gotten-to-be-standard push-me-pull-you with the PROGMEM attribute and avr-gcc. I battle this regularly myself as I go between different avr code bases which use different variations on this, each variation intended to appease some specific version of the gcc underlying avr-gcc. (If you turn on the detailed logging in the Arduino app, it’s not at all unusual to see compile time warnings relating to progrmem as Arduino library code is compiled in. But those are just warnings and not errors such as here.)
I tend to get the best mileage out of this particular incantation,
const static PROGMEM unsigned int fiveLetterPosArray[] = {363, … };
You might also try
prog_int16_t fiveLetterPosArray[] PROGMEM = {363, …};
which is the “official” Arduino way (http://arduino.cc/en/Reference/PROGMEM). But that tends to just mean you get a warning and not an error.
