Declare S9 example functions only if s9api on deploy-time class path

[jcflack]

The Maven build has long offered an optional -Psaxon-examples profile; without it, the examples jar gets built without the S9 example, and can be deployed in a database without the Saxon jar loaded. With the profile active, the jar is built with the S9 example included, and then that jar can't be loaded (with deploy => true) unless the database already has a Saxon jar installed and on the class path, even if there is no need for the S9 examples. That's an inconvenient restriction on later use of the examples jar based on a choice made at build time.

Use conditional execution in the deployment descriptor so that even when the examples jar is built with the S9 examples included, they are only deployed if a Saxon API class is found on the class path at deploy time. That way, the examples can be built with the saxon-examples profile active and still freely used without loading a Saxon jar if those examples are not of interest.

Some inconvenient situations, though less likely ones, can still arise. If the examples jar is installed when no Saxon jar is on the class path (and so does not declare the S9 example functions), but later remove_jar is called (with undeploy => true) when for some reason there is a Saxon jar on the class path, then the undeploy actions will detect the Saxon classes and try to drop the S9 functions that had not been declared. But this is a rare "so don't do that" case that is easily avoided, nothing like the serious inconvenience of never being able to use the examples jar without installing Saxon first, depending on how the jar was built.