threads: use named variable for temporary cleanup to allow C++ calls in win32

This would allow compilation of DBMSDIOutput on Windows and also allow using it in the projectm plugin.

There doesn't seem to be any locally cascaded calls to vlc_cleanup_push/mutex_cleanup_push so there will not be any variable shadowing. It's unlikely the cascading would be used in a single function anyway.

include/vlc_threads.h

@@ -1024,25 +1024,18 @@ struct vlc_cleanup_t
     void          *data;
 };
 
-# ifndef __cplusplus
 /* This macros opens a code block on purpose: It reduces the chance of
  * not pairing the push and pop. It also matches the POSIX Thread internals.
  * That way, Win32 developers will not accidentally break other platforms.
  */
 # define vlc_cleanup_push( routine, arg ) \
     do { \
-        vlc_control_cancel(&(vlc_cleanup_t){ NULL, routine, arg })
+        vlc_cleanup_t cancel_cleanup = { NULL, routine, arg }; \
+        vlc_control_cancel( &cancel_cleanup )
 
 #  define vlc_cleanup_pop( ) \
         vlc_control_cancel (NULL); \
     } while (0)
-# else
-#  define vlc_cleanup_push(routine, arg) \
-    static_assert(false, "don't use vlc_cleanup_push in portable C++ code")
-#  define vlc_cleanup_pop() \
-    static_assert(false, "don't use vlc_cleanup_pop in portable C++ code")
-# endif
-
 #endif /* !LIBVLC_USE_PTHREAD_CLEANUP */
 
 static inline void vlc_cleanup_lock (void *lock)