autotools: warning: deprecated directive: ‘%error-verbose’, use ‘%define parse.error verbose’

`alocal` warns:

```text
pbc/parser.y:20.1-14: warning: deprecated directive: ‘%error-verbose’, use ‘%define parse.error verbose’ [-Wdeprecated]
   20 | %error-verbose
      | ^~~~~~~~~~~~~~
      | %define parse.error verbose
pbc/parser.y: warning: fix-its can be applied.  Rerun with option '--update'. [-Wother]
```