Nashorn build targets version 9 source

The nashorn java code requires a somewhat convoluted compilation. As a result, we use a special java compiler setup, GENERATE_NEWBYTECODE_DEBUG. This explicitly lists -source 9 -target 9, which generates this warning:

warning: [options] bootstrap class path not set in conjunction with -source 1.9

when building in jdk10. I assume this is just a mistake, and that it should really target 10. (Or, to always be in sync, perhaps even $(VERSION_MAJOR)).