Set default for ARDUINOJSON_ENABLE_PROGMEM to 1 on AVR

Ported from 082ae69e86

extras/conf_test/avr.cpp

@@ -1,5 +1,7 @@
 #include <ArduinoJson.h>
 
+static_assert(ARDUINOJSON_ENABLE_PROGMEM == 1, "ARDUINOJSON_ENABLE_PROGMEM");
+
 static_assert(ARDUINOJSON_USE_LONG_LONG == 0, "ARDUINOJSON_USE_LONG_LONG");
 
 static_assert(ARDUINOJSON_SLOT_OFFSET_SIZE == 1,

src/ArduinoJson/Configuration.hpp

@@ -130,9 +130,13 @@
 #    define ARDUINOJSON_ENABLE_ARDUINO_PRINT 0
 #  endif
 
-// Disable support for PROGMEM
+// Enable PROGMEM support on AVR only
 #  ifndef ARDUINOJSON_ENABLE_PROGMEM
-#    define ARDUINOJSON_ENABLE_PROGMEM 0
+#    ifdef __AVR__
+#      define ARDUINOJSON_ENABLE_PROGMEM 1
+#    else
+#      define ARDUINOJSON_ENABLE_PROGMEM 0
+#    endif
 #  endif
 
 #endif  // ARDUINO