Recent `bison` warns for `%pure-parser`. We kept it since MacOS only
provide ancient `bison`, but the warning is noisy and there's no hope
that Apple will upgrade `bison`. MacOS users must install the newer
version of `bison`, by typing `brew install bison` for example.

Note that `brew` does not overwrite the `bison` execution path
automatically, so you need to update your `.bash_profile` as instructed
by `brew`.