[Bug c/60654] format warnings don't work with PROGMEM/PSTR

[code at dawg dot eu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60654

--- Comment #2 from Martin Nowak <code at dawg dot eu> ---
Those are macros to declare strings in read-only flash memory.
http://www.nongnu.org/avr-libc/user-manual/group__avr__pgmspace.html