If Catala is compiled without Z3, trying to run it with the backend `Proof` will
yield:
```
[ERROR] This instance of Catala was compiled without Z3 support.
```
and return 124
Note that this doesn't change the `make depends`, opam file or CI to account for it,
it just enables it at the build-system level.
There are also no hooks at this moment to have Catala self-document the options
whith which it was compiled (e.g. in the `--help` screen). But that could be
added in a more general way later, it's probably not really needed yet.
