[libcxx] [llvm] [libc++] Refactor the configuration macros to being always defined (PR #112094)

[Louis Dionne via llvm-commits]

ldionne wrote:

As you mentioned, checking libc++ internal macros is not something we support. If we supported that, we couldn't make any modifications to the code base without breaking people.

Perhaps you could do something along those lines instead?

```c++
if constexpr (std::is_same_v<std::thread::native_handle_type, pthread_t>) {
  // assume pthread
}
```

https://github.com/llvm/llvm-project/pull/112094