Meson nitpicks

I originally wrote meson build definitions for pixman nearly 2 years ago then never took time to upstream them :(

Anyway, the port that was upstreamed looks very complete, congrats, these are the only nitpicks I could find when comparing. I will say that my port was making extensive use of dicts and was a bit more elegant / concise thanks to that, in particular wrt the simd checks, but functionally both ports are equivalent afaict :)