Thank you for your reply. I've spent some years with Python and indeed with the jscompiler and closure library. But with Java, alas, none at all.
As I understood it, I got this warning ("bootstrap class path not set in conjunction with -source 1.7", which was then raised up to an error), because I hadn't pointed ant (presumably through $JAVA_HOME) to a working 1.7 JDK. And it wants the 1.7 JDK, since build.xml specifies both source and target = 1.7.
What's wasn't quite clear was build.xml needed to specify the source JDK and target JVM for its three compilation targets (closure-stylesheets, closure compiler + templates, and plovr itself). Is there anything about these projects that precludes using Java 8? Or were the 1.7 attributes set just to prevent building with a JDK < 1.7?
And if plovr can be built with Java 8, is there any change I can offer to build.xml so that we can build plovr with ant without having to alter build.xml? Removing the source / target attributes from these there <javac> elements seems to do the trick, but I know too little about Ant and building Java in general to think that's actually a good idea. What would be?