C11 required to use ATOMIC_VAR_INIT to statically initialize
atomic objects with static storage duration. Yet this macro
was unsuitable for initializing structures [1] and was actually
unneeded for all known implementations (this includes our
compatibility fallback implementations which simply wrap the value
in parentheses: #define ATOMIC_VAR_INIT(value) (value)).
Therefore C17 deprecated the macro and C23 actually removed it [2].

Since commit 5ff0eb34 we default
to C17 if the compiler supports it; Clang warns about ATOMIC_VAR_INIT
in this mode. Given that no implementation ever needed this macro,
this commit stops using it to avoid this warning.

[1]: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr_485
[2]: https://en.cppreference.com/w/c/atomic/ATOMIC_VAR_INITSigned-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>