Fix incorrect usage of COBJMACROS.

diff --git a/include/mapix.h b/include/mapix.h
index 189c159..7db100a 100644
--- a/include/mapix.h
+++ b/include/mapix.h
@@ -135,7 +135,7 @@
 DECLARE_INTERFACE_(IMAPISession,IUnknown) { IMAPISession_METHODS };
 #undef INTERFACE
 
-#ifdef COBJMACROS
+#if !defined(__cplusplus) || defined(CINTERFACE)
         /*** IUnknown methods ***/
 #define IMAPISession_QueryInterface(p,a,b)          (p)->lpVtbl->QueryInterface(p,a,b)
 #define IMAPISession_AddRef(p)                      (p)->lpVtbl->AddRef(p)