Variables by these names are only used from the local wscript and when
running "waf configure", which already for other reasons only can run at
the top-level.
These variables are thus not mandatory and not used.
Most wscripts use
#!/usr/bin/env python
Use that consistently.
The wscripts are not executed directly and do not need the shebang path,
but it might guide editors and other tools to recognize the files as
being Python-ish.
