Better behavior when the targets of a rule already exist in the source tree

[ghost]

At the moment, if the targets of a rule are already present in the source tree, jbuilder ignores the rule. This is convenient for configure scripts: one can have the configure script generate a configuration file based on command line arguments supplied by the user and a rule to generate a default one. However, it is error prone in other cases. Especially when they are left-over files from a previous build system in the source tree.

This PR changes Jbuilder to emit a warning when such a case happens. The user now has to add (fallback) to the rule to specify that the rule only applies when the targets do not already exist in the source tree. For instance:

;; generate an empty lwt_config if it doesn't exist.
;; the discover script will generate defaults as appropriate.
(rule
 ((targets (lwt_config))
  (fallback)
  (action (with-stdout-to lwt_config (echo "")))))

Additionally, it is an error if only a subset of the targets are present.

This PR fixes #187 and #148.

[ghost]

One limitation of the current design is that one cannot turn a (ocamllex ...) stanza into a fallback rule. One way to make this more generic would be to add a fallback stanza that would act as a modifier for other stanzas:

(fallback
  (rule (...))
  (ocamllex (...))
  ...
)

However, it seems that the only use of fallback rules is for default configuration files. So it seems better to keep things simple for now.