Add tools.jar to classpath only on Windows and Linux.

Mac OS X bundles tools.jar with classes.jar so there is no need to add
tools.jar. But on Windows and Linux, tools.jar is not bundled in rt.jar, so
tools.jar must be added to classpath separately.

See http://stackoverflow.com/questions/12409847/how-to-add-tools-jar-as-a-dynamic-dependency-in-sbt-is-it-possible