Issue #488: use JSON_EXPORT on functions so they are properly exported on Windows.

json_object.h

@@ -33,11 +33,13 @@
 #define JSON_C_CONST_FUNCTION(func) func
 #endif
 
+#ifndef JSON_EXPORT
 #if defined(_MSC_VER) 
 #define JSON_EXPORT __declspec(dllexport)
 #else
 #define JSON_EXPORT extern
 #endif
+#endif
 
 #include <stddef.h>
 #include "json_inttypes.h"