Allow using PROGMEM outside of Arduino (fixes #1903)

Ported from 40daf56b5a

src/ArduinoJson.hpp

@@ -13,7 +13,8 @@
 // Include Arduino.h before stdlib.h to avoid conflict with atexit()
 // https://github.com/bblanchon/ArduinoJson/pull/1693#issuecomment-1001060240
 #if ARDUINOJSON_ENABLE_ARDUINO_STRING || ARDUINOJSON_ENABLE_ARDUINO_STREAM || \
-    ARDUINOJSON_ENABLE_ARDUINO_PRINT || ARDUINOJSON_ENABLE_PROGMEM
+    ARDUINOJSON_ENABLE_ARDUINO_PRINT ||                                       \
+    (ARDUINOJSON_ENABLE_PROGMEM && defined(ARDUINO))
 #  include <Arduino.h>
 #endif
 

src/ArduinoJson/Deserialization/Readers/FlashReader.hpp

@@ -4,7 +4,7 @@
 
 #pragma once
 
-#include <Arduino.h>
+#include <ArduinoJson/Polyfills/pgmspace.hpp>
 
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 

src/ArduinoJson/Polyfills/pgmspace.hpp

@@ -4,7 +4,13 @@
 
 #pragma once
 
-#include <Arduino.h>
+#ifdef ARDUINO
+#  include <Arduino.h>
+#else
+// Allow using PROGMEM outside of Arduino (issue #1903)
+class __FlashStringHelper;
+#  include <avr/pgmspace.h>
+#endif
 
 #include <ArduinoJson/Configuration.hpp>
 #include <ArduinoJson/Namespace.hpp>

src/ArduinoJson/Strings/Adapters/FlashString.hpp

@@ -4,8 +4,6 @@
 
 #pragma once
 
-#include <Arduino.h>
-
 #include <ArduinoJson/Polyfills/pgmspace.hpp>
 #include <ArduinoJson/Strings/StringAdapter.hpp>