Update declarations for PROGREM, to build with avr-gcc 4.8.1 / Arduino 1.5.7

- PROGMEM error: avr-gcc 4.8.1 reports:
  variable ... must be const in order to be put into read-only section by means of ‘__attribute__((progmem))’
  and the fix is to make the variable 'const'

- also add ifdef to support non-US date format (D/M/Y)