Build platform, run platform, and Java 1.8

[Joseph Kessselman]

While advising me on the Maven stuff, Alex Kriegsch pointed out that the javadoc taglet we've been using depended on APIs which were replaced after Java 8. He very kindly rattled off a version of that code which will run on Java 9 and later, and showed us how to make the build conditional so it would build one or the other depending on which platform we were building on.

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This only affects build, and javadoc in particular. We're still targeting Java 8 as our minimum for running Xalan. This raises the question of whether we want to promise that our build (as opposed to our product) is Java 8 compatible. There's certainly something to be said for building on a more recent Java with 1.8 set as the target. We *could* advance our official minimum build platform without having to advance the product. But it also raises the question of how long we want to promise Java 8 support in the product. That's pretty darned archaic at this point; I've heard claims that sometime in the next release or two Java is going to drop 1.8 as a compile target. We may want to consider raising our minimum requirement as well, or at least run that idea past the users we're in touch with and see if any of them will scream. Surfacing it here first in case someone knows of a good reason we shouldn't open that can of worms... Caveat: Opinionated old geezer with overcompensated writer's block. May be redundant, verbose, prolix, sesquipedalian, didactic, officious, or redundant. Feel free to call him on it.

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