ability to consistently apply a toolchain across plugins

for things like the JDK, it may be desirable to compile, jar etc. across the board with a particular version of the toolchain, other than the one currently executing. It would be good to be able to configure this location in the settings.xml and have the plugins use it, forking the compiler, using a particular runtime, and generating an appropriate manifest.

This is likely to be relevant to other languages too.