Meson 0.56.2 and Python39 --> dav1d failing

[Ken Cunningham]

> On Feb 3, 2021, at 11:49 AM, Craig Treleaven <ctreleaven at macports.org> wrote:
> 
> But configure still failed on 10.7 and 10.8:

Oh no! It looked so great! I was really learning some things there. Perhaps it can be tweaked still.

If not, I guess we can still use the compiler_blacklist_versions approach and blacklist all the clangs < whatever 10.10 comes with.

Ken