Make warning 3 (deprecated feature) non-fatal for the mezzo build.

The fatal warning for deprecated features causes the mezzo build to fail with OCaml 4.02.0:

```
# File "utils/Utils.ml", line 74, characters 10-23:
# Warning 3: deprecated: String.create
# File "utils/Utils.ml", line 1:
# Error: Some fatal warnings were triggered (1 occurrences)
# Command exited with code 2.
# Makefile:35: recipe for target 'all' failed
```

0695745 adds a patch to make the warning non-fatal.

/cc @protz