[Boost version] Bad detection of `std::aligned_alloc` for XCode 12.2 with `-std=c++1z`

[joaquintides]

As shown here: https://drone.cpp.al/boostorg/unordered/1601/52/2

In file included from ./boost/asio.hpp:20:
In file included from ./boost/asio/any_completion_executor.hpp:22:
In file included from ./boost/asio/execution.hpp:19:
In file included from ./boost/asio/execution/any_executor.hpp:24:
In file included from ./boost/asio/detail/executor_function.hpp:19:
In file included from ./boost/asio/detail/handler_alloc_helpers.hpp:19:
./boost/asio/detail/memory.hpp:107:15: error: no member named 'aligned_alloc' in namespace 'std'; did you mean simply 'aligned_alloc'?
  void* ptr = std::aligned_alloc(align, size);
              ^~~~~~~~~~~~~~~~~~
              aligned_alloc
/Applications/Xcode-12.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/malloc/_malloc.h:50:10: note: 'aligned_alloc' declared here
void    *aligned_alloc(size_t __alignment, size_t __size) __result_use_check __alloc_size(2) __OSX_AVAILABLE(10.15) __IOS_AVAILABLE(13.0) __TVOS_AVAILABLE(13.0) __WATCHOS_AVAILABLE(6.0);
         ^
1 error generated.

This issue seems to be directly related to boostorg/asio@8b80ad7. I know Boost.Asio is generated by script from standalone Asio, but the code in chriskohlhoff/asio/blob/master/include/asio/detail/config.hpp does not include the change, so I'm obviously not getting entirely how these two libraries relate.