C++20 broke two things in the prior bump:
1. std::allocator no longer exposes pointer/const_pointer/reference/
const_reference member typedefs, and the 2-arg allocate(n, hint) was
removed. Both secure_allocator and zero_after_free_allocator inherited
these from std::allocator. Define the typedefs ourselves and switch
the secure_allocator allocate() to the single-arg form.
2. Bundled src/leveldb uses `std::memory_order::memory_order_relaxed`
which was valid in C++17 but became a hard error in C++20 (memory_order
is now a scoped enum class — the values are at namespace scope or
memory_order::relaxed, not memory_order::memory_order_relaxed). LevelDB
itself only needs C++11, so pin its targets to C++17 in BuildLevelDB.cmake
instead of patching vendored code.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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