I chose a name that would avoid collisions with anything a user might potentially throw into their environment, so if the var "OS" is fairly standard is a reason to avoid using it. Also, going with our own env var allows us to stay in perfect synchronization with the build's #ifdef WIN32 ... and whatever #ifdefs may come in the future for new OSes. If there is already an environment variable that does that for us, I would rather use that, but I haven't found it.
Perhaps, I didn't make myself clear. Your solution is perfectly adapted to our needs.
But all Windows since 2000 already have an environment variable OS=Windows_NT. So, if env OS is defined and equal Windows_NT, this had to be Windows.
May we use it in our case? I don't insist, just asking.