Document PYTHON_CROSSENV variable

[virtuald]

My setup.py conditionally runs an executable -- when cross compiling, it turns it off. I was looking for _PYTHON_HOST_PLATFORM, but crossenv deletes that?

Now I'm checking for PYTHON_CROSSENV... but I'd be happier if it were a documented way to detect crossenv.

[benfogle]

Crossenv also sets sys.cross_compiling, which can be used like if getattr(sys, 'cross_compiling', False): ...

Either one of those should work for you, but you're absolutely right: both of those should be documented.

[benfogle]

Closed by #59