[3.13] gh-121487: Fix deprecation warning for ATOMIC_VAR_INIT in mimalloc (gh-121488) (#121504)

(cherry picked from commit 31873be)

Co-authored-by: Marc Mueller <30130371+cdce8p@users.noreply.github.com>

Author: miss-islington

Diff for: Include/internal/mimalloc/mimalloc/atomic.h

@@ -23,7 +23,9 @@ terms of the MIT license. A copy of the license can be found in the file
 #define  _Atomic(tp)            std::atomic<tp>
 #define  mi_atomic(name)        std::atomic_##name
 #define  mi_memory_order(name)  std::memory_order_##name
-#if !defined(ATOMIC_VAR_INIT) || (__cplusplus >= 202002L) // c++20, see issue #571
+#if (__cplusplus >= 202002L)    // c++20, see issue #571
+ #define MI_ATOMIC_VAR_INIT(x)  x
+#elif !defined(ATOMIC_VAR_INIT)
  #define MI_ATOMIC_VAR_INIT(x)  x
 #else
  #define MI_ATOMIC_VAR_INIT(x)  ATOMIC_VAR_INIT(x)
@@ -39,7 +41,9 @@ terms of the MIT license. A copy of the license can be found in the file
 #include <stdatomic.h>
 #define  mi_atomic(name)        atomic_##name
 #define  mi_memory_order(name)  memory_order_##name
-#if !defined(ATOMIC_VAR_INIT) || (__STDC_VERSION__ >= 201710L) // c17, see issue #735
+#if (__STDC_VERSION__ >= 201710L) // c17, see issue #735
+ #define MI_ATOMIC_VAR_INIT(x) x
+#elif !defined(ATOMIC_VAR_INIT)
  #define MI_ATOMIC_VAR_INIT(x) x
 #else
  #define MI_ATOMIC_VAR_INIT(x) ATOMIC_VAR_INIT(x)

Diff for: Misc/NEWS.d/next/Build/2024-07-08-14-01-17.gh-issue-121487.ekHmpR.rst

@@ -0,0 +1 @@
+Fix deprecation warning for ATOMIC_VAR_INIT in mimalloc.