github.com/PaulStoffregen/cores/blob/master/teensy3/avr/pgmspace.h
github.com/arduino/ArduinoCore-sam/blob/master/cores/arduino/avr/pgmspace.h
ESP8266
pgm_read_byte() does not work well without delay() #3140
Microchip C18
How to port pgm_read_byte macro (AVR-GCC) to Mircrochip C18 Compiler?