Attributing the (base) class deprecated does not lead to as wide
coverage of the deprecation warnings as we hoped, but attributing
functions seems to work. Let's try marking the constructors as
deprecated (on a small test it seems to work).

While in principle annotating the entire class would be the proper
thing to do, with gcc the constructor annotation appears to work
better for identifying the deriving classes that need to be migrated
to thread-friendly base classes.