[SmileyAG]

Alongside of #585 PR, this should addresses #584 issue too.

_WIN32_WINNT=0x0601 and WINVER=0x0601 is not required, since v143 toolset sets subsystem version to 6.00 (i.e. Windows Vista) by default, example of latest release with dumpbin /headers:

OPTIONAL HEADER VALUES
             10B magic # (PE32)
           14.44 linker version
          1B5C00 size of code
        13C0F400 size of initialized data
               0 size of uninitialized data
          17B609 entry point (1017B609)
            1000 base of code
          1B7000 base of data
        10000000 image base (10000000 to 23DC8FFF)
            1000 section alignment
             200 file alignment
            6.00 operating system version
            0.00 image version
            6.00 subsystem version
               0 Win32 version

[SmileyAG]

To make a final comment on it, BXT in its current state does not use runtime libraries from Boost projects (Boost.Atomic, Boost.Filesystem and etc) and rely completely on header-only libraries from Boost.
After all, most of Boost libraries is implemented as header-only.

Downgrading Rust to 1.77.2 version and keep v143 toolset (i.e. not upgrading to v145) is pretty much enough to fully support Windows 7 in current state.

So I'll keep this PR open, however this is feels extra.