#if !defined(_LIBCPP_HAS_NO_THREADS)
    #if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
        ...
    #elif defined(_LIBCPP_HAS_THREAD_API_C11)
        ...
    #elif !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
        ...
        #if defined(_M_IX86) || defined(__i386__) || defined(_M_ARM) || defined(__arm__)
            typedef void* __libcpp_recursive_mutex_t[6];
        #elif defined(_M_AMD64) || defined(__x86_64__) || defined(_M_ARM64) || defined(__aarch64__)
            typedef void* __libcpp_recursive_mutex_t[5];
        #else
            #error Unsupported architecture
        #endif
    #endif
    ...
#endif