Older versions of GCC (<4.9) and LLVM (<3.6) do not have __atomic_load_n
which is part of the C11 standard. Fix by falling back to
__sync_fetch_and_add with a value of 0 when __atomic_load_n is not
available.
Fixes#423
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>