Meson 0.56.2 and Python39 --> dav1d failing

[Joshua Root]

On 2021-2-3 23:49 , Ken Cunningham wrote:
> dav1d is failing on 10.7 and 10.8 on the configure test for atomic.
> 
> it's building on 10.6 with clang 9.0, so probably needs to blacklist clangs older than 10.9's clang.
> 
> nothing to do with meson, looks to me...

It's claimed to be written in C99, but requires either C11's stdatomic.h 
or GCC extensions. Hmmm.

- Josh