Added a clear error message when compiled as C (issue #629)

CHANGELOG.md

@@ -5,6 +5,7 @@ HEAD
 ----
 
 * Added `JsonVariant::operator|` to return a default value (see bellow)
+* Added a clear error message when compiled as C instead of C++ (issue #629)
 * Rewrote example `JsonHttpClient.ino` (issue #600)
 
 > ### How to use the new feature?

src/ArduinoJson.h

@@ -4,6 +4,14 @@
 
 #pragma once
 
+#ifdef __cplusplus
+
 #include "ArduinoJson.hpp"
 
 using namespace ArduinoJson;
+
+#else
+
+#error ArduinoJson requires a C++ compiler, please change file extension to .cc or .cpp
+
+#endif