Don't use _Atomic in C99 mode

```
/github/workspace/src/ext/digest/blake3/blake3_dispatch.c:108:5: error: '_Atomic' is a C11 extension [-Werror,-Wc11-extensions]
      108 |     ATOMIC_INT g_cpu_features = UNDEFINED;
          |     ^
/github/workspace/src/ext/digest/blake3/blake3_dispatch.c:34:20: note: expanded from macro 'ATOMIC_INT'
       34 | #define ATOMIC_INT _Atomic int
          |                    ^
1 error generated.
```

Author: byroot

ext/digest/blake3/extconf.rb

@@ -78,6 +78,18 @@ def blake3_have_isa?(name, flag, snippet)
 
 have_header("sys/cdefs.h")
 
+checking_for("C11 atomics") do
+  ok = try_compile(<<~C)
+    static _Atomic int atomic_int = 0;
+    static int t(void) {return atomic_int;}
+    #{MAIN_DOES_NOTHING 't'}
+  C
+  unless ok
+    $defs << "-DBLAKE3_ATOMICS=0"
+  end
+  ok
+end
+
 $preload = %w[digest]
 
 create_makefile("digest/blake3")