Add (mode fallback) to installConfig.ml rule

[ghost]

To make the behaviour explicit when the installConfig.ml exist in the source tree because configure.ml generated it. Currently, we get the following warning:

$ dune build -p camomile
File "Camomile/dune", line 2, characters 0-96:
2 | (rule (with-stdout-to installConfig.ml
3 |        (echo "let share_dir = \"/usr/share/camomile\"")))
Warning: File installConfig.ml is both generated by a rule and present in the source tree.
As a result, the rule is currently ignored, however this will become an error in the future.
To keep the current behavior and get rid of this warning, add a field (fallback) to the rule.

[Mic92]

Would be cool to get a new release that has this build fix included.