Defining `-Xlint:-options` (presumably to suppress the warning) instead of explicitly setting a `-bootclasspath` for a JDK 6-based "rt.jar" makes me a little nervous that we might inadvertently reference methods in code that don't exist prior to JDK 7 and not catch it.  That said, I don't know that I have a better suggestion since hard-coding a `-bootclasspath` in this file doesn't seem practical either.

I'd still be in favor as a final sanity check to at least have the *release* jobs build artifacts with a JDK 6 rather than possibly cross-compiling from a JDK 7 or newer.  Been burned on cross-compile JDK stuff in the past, so I'm a bit sensitive on this stuff.