Message 140252 - Python tracker

When I built the documentation on Win7, it failed with a SyntaxError, and it took some digging to find the reason why.  I was hoping that issuing this warning would save others the trouble.

I agree: on Windows, PYTHON is rarely set.  However, look in make.bat: you'll see that when PYTHON is not already set, it's set to ..\pcbuild\python, which is Python 3.