Fix incorrect usage of COBJMACROS.

diff --git a/include/vfw.h b/include/vfw.h
index 6e62f37..94c323c 100644
--- a/include/vfw.h
+++ b/include/vfw.h
@@ -1007,7 +1007,7 @@
 DECLARE_INTERFACE_(IAVIStream, IUnknown) { IAVIStream_METHODS };
 #undef INTERFACE
 
-#ifdef COBJMACROS
+#if !defined(__cplusplus) || defined(CINTERFACE)
 /*** IUnknown methods ***/
 #define IAVIStream_QueryInterface(p,a,b) (p)->lpVtbl->QueryInterface(p,a,b)
 #define IAVIStream_AddRef(p)             (p)->lpVtbl->AddRef(p)
@@ -1121,7 +1121,7 @@
 DECLARE_INTERFACE_(IAVIStreaming, IUnknown) { IAVIStreaming_METHODS };
 #undef INTERFACE
 
-#ifdef COBJMACROS
+#if !defined(__cplusplus) || defined(CINTERFACE)
 /*** IUnknown methods ***/
 #define IAVIStreaming_QueryInterface(p,a,b) (p)->lpVtbl->QueryInterface(p,a,b)
 #define IAVIStreaming_AddRef(p)             (p)->lpVtbl->AddRef(p)
@@ -1145,7 +1145,7 @@
 DECLARE_INTERFACE_(IAVIEditStream,IUnknown) { IAVIEditStream_METHODS };
 #undef INTERFACE
 
-#ifdef COBJMACROS
+#if !defined(__cplusplus) || defined(CINTERFACE)
 /*** IUnknown methods ***/
 #define IAVIEditStream_QueryInterface(p,a,b) (p)->lpVtbl->QueryInterface(p,a,b)
 #define IAVIEditStream_AddRef(p)             (p)->lpVtbl->AddRef(p)
@@ -1194,7 +1194,7 @@
 DECLARE_INTERFACE_(IAVIFile,IUnknown) { IAVIFile_METHODS };
 #undef INTERFACE
 
-#ifdef COBJMACROS
+#if !defined(__cplusplus) || defined(CINTERFACE)
 /*** IUnknown methods ***/
 #define IAVIFile_QueryInterface(p,a,b) (p)->lpVtbl->QueryInterface(p,a,b)
 #define IAVIFile_AddRef(p)             (p)->lpVtbl->AddRef(p)
@@ -1242,7 +1242,7 @@
 DECLARE_INTERFACE_(IGetFrame,IUnknown) { IGetFrame_METHODS };
 #undef INTERFACE
 
-#ifdef COBJMACROS
+#if !defined(__cplusplus) || defined(CINTERFACE)
 /*** IUnknown methods ***/
 #define IGetFrame_QueryInterface(p,a,b) (p)->lpVtbl->QueryInterface(p,a,b)
 #define IGetFrame_AddRef(p)             (p)->lpVtbl->AddRef(p)