Fix compile on older versions of GCC and CLANG (#427)
Fix compile on older versions of GCC and CLANG (#427)

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 <78036c9b69b887700d5846f26a788d53b201ffbb@gmail.com>
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