make sure all needed api are exported

Author: SilvanScherrer

include/json/config.h

@@ -35,6 +35,8 @@
 #if defined(_MSC_VER) || defined(__MINGW32__)
 #define JSON_API __declspec(dllexport)
 #define JSONCPP_DISABLE_DLL_INTERFACE_WARNING
+#elif defined(__OS2__)
+#define JSON_API __declspec(dllexport)
 #elif defined(__GNUC__) || defined(__clang__)
 #define JSON_API __attribute__((visibility("default")))
 #endif // if defined(_MSC_VER)