[STDC++COMPAT] Add rand_s for GCC 13 and pre-NT6 builds

GCC 13 STL requires this.

sdk/lib/gcc-compat/CMakeLists.txt

@@ -7,6 +7,11 @@ list(APPEND SOURCE
     __throw_out_of_range_fmt.cpp
     StringCchCopyNA.c)
 
+if(DLL_EXPORT_VERSION LESS 0x600)
+    list(APPEND SOURCE
+        rand_s.c)
+endif()
+
 add_library(stdc++compat ${SOURCE})
 set_target_cpp_properties(stdc++compat WITH_EXCEPTIONS)
 add_dependencies(stdc++compat xdk)

sdk/lib/gcc-compat/rand_s.c

@@ -0,0 +1,23 @@
+/*
+ * PROJECT:     GCC C++ support library
+ * LICENSE:     MIT (https://spdx.org/licenses/MIT)
+ * PURPOSE:     Workaround for missing rand_s on pre-NT6 builds
+ * COPYRIGHT:   Copyright 2026 Timo Kreuzer <timo.kreuzer@reactos.org>
+ */
+
+#include <stdlib.h>
+
+errno_t __cdecl rand_s(_Out_ unsigned int *RandomValue)
+{
+    if (!RandomValue)
+        return EINVAL;
+
+    *RandomValue = rand();
+    return 0;
+}
+
+#ifdef _M_IX86
+void* _imp__rand_s = rand_s;
+#else
+void* __imp_rand_s = rand_s;
+#endif