Is there a feature macro for cross-process-capable atomics?

[Jacob Champion]

Hi all,

I'd like to be able to use the apr_atomic primitives for cross-process 
work in shared memory, but as far as I can see, not every implementation 
is guaranteed to work across processes (for example, the generic 
implementation uses apr_thread_mutex*).

I'd be fine falling back to a heavyweight implementation if a platform 
truly needed it, but I can't find a way to "query" characteristics of 
the atomics implementation, so the fallback unfortunately has to be the 
default. Is there a feature macro I'm missing somewhere?

--Jacob