Removing ATOMIC_REFCOUNT remainders from build system?

[Rik]

jwe,

Since you re-built the atomic operations atop C++ std::atomic, there
doesn't seem to be any use for OCTAVE_ENABLE_ATOMIC_REFCOUNT.  Should the
remaining vestiges in configure.ac, toplev.cc, oct-conf-post.in.h,
mk-octave-config-h.sh be removed?

Alternatively, we could re-instate the functionality in oct-refcount.h

#if defined OCTAVE_ENABLE_ATOMIC_REFCOUNT
    std::atomic<T> m_count;
#else
  T m_count;
#endif

This would mostly be of use in determining the penalty we pay for
cross-thread communication.  It might be useful if we eventually attempt an
optimization such that only objects which are traveling cross-thread use
the atomic interface.  But, this is speculative so I'm also fine with just
removing it.

--Rik