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As usual, we use C11 semantics, and emulate it if <stdatomic.h> is not
available.
It's a bit messy with __sync_val_compare_and_swap(). We assume it has
"strong" semantics (it can't fail sporadically), but I'm not sure if
this is really the case. On the other hand, weak semantics don't seem to
be possible, since the builtin can't distinguish between the two failure
cases that could occur. Also, to match the C11 interface, use of gcc
builtins is unavoidable. Add a check to the build system to make sure
the compiler supports them (although I don't think there's any compiler
which supports __sync_*, but not these extensions).
Needed for the following commit.
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