Fix GCC 9.4 build: gate std::atomic<shared_ptr> on C++20 feature macro

[jfsantos]

Problem

This project fails to build under GCC 9.4.0 because NAM/wavenet/slimmable.h uses std::atomic<std::shared_ptr<T>>, a C++20 library feature that libstdc++ only supports from GCC 12.

The header already had a fallback to the deprecated std::atomic_* free-function overloads for shared_ptr, but it was keyed on #ifdef _LIBCPP_VERSION. GCC 9.4 uses libstdc++ (not libc++), so it took the #else branch and required the unsupported std::atomic<std::shared_ptr<>>.

Fix

Key the selection on the standard C++20 feature-test macro __cpp_lib_atomic_shared_ptr instead of on a specific standard library:

#if defined(__cpp_lib_atomic_shared_ptr) && __cpp_lib_atomic_shared_ptr >= 201711L
  #define NAM_HAS_ATOMIC_SHARED_PTR 1
#else
  #define NAM_HAS_ATOMIC_SHARED_PTR 0
#endif

• GCC 12+ (macro defined) → unchanged, uses the member std::atomic<std::shared_ptr<>> path.
• libc++ (macro never defined — feature still unimplemented) → unchanged, uses the free-function fallback exactly as before.
• GCC 9.4 + libstdc++ (macro undefined) → now correctly takes the free-function fallback instead of failing to compile.

No behavior change for any compiler that previously built.

Files changed

• NAM/wavenet/slimmable.h — feature-macro guard, conditional include, member declaration.
• NAM/wavenet/slimmable.cpp — the four staging accessor definitions.