waf.python configurable?

[Zhenfu Shi via macports-dev]

Hi all,
I’m working on mpv portfile but I couldn’t find a way to change {waf.python}, 
the python executable needed for waf. It’s currently failing to configure on 
buildbots. In https://trac.macports.org/ticket/57927 
<https://trac.macports.org/ticket/57927> the original maker of waf portgroup 
said it is configurable. Anyone knows how?

Thanks,
Zhenfu