Mismatched _WIN32_WINNT version between boost::log and boost::asio on Windows >= 8 environments.

I had to hunt down a link error today, and I am taking some time to report it here.

On a Windows 10 environment, I am building a project with two distinct libraries:
* `Library A` links against `boost::log`
* `Library B` links against `boost::asio` and `library A`

`Library B` fails to link, with the following linker error:
> B.lib: error LNK2038: mismatch detected for 'boost_log_abi': value 'v2s_mt_nt6' doesn't match v
alue 'v2s_mt_nt62' in A.lib

As far as I understand, this is because `boost::log` determines a "default" Windows version of **0x0602** ([here](https://github.com/boostorg/log/blob/develop/include/boost/log/detail/config.hpp#L362)) when compiling on Windows >= 8,
whereas `boost::asio` assumes a default Windows version of **0x0601** ([here](https://github.com/boostorg/asio/blob/boost-1.81.0/include/boost/asio/detail/config.hpp#L1562)).

Users can solve this issue by setting an explicit Windows API version, yet it could be preferable if it also linked with default values.