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When run on windows in a venv, PYTHON_LIBRARY pointet to a non-existant
location in the virtual environment directory.

This has been fixed by testing if the path exists and, if not, trying
an alternative path, relative to the PYTHON_INCLUDE_DIR.

If the alternative path doesn't exit as well, an error will be raised.

Also:
- Update documented result variables
- Remove long-ago-deprecated variable

This prevented `DOC(...)` calls with 5 arguments to resolve correctly.