Handle missing tools.jar

[dspinellis]

According to this SO answer "Tools.jar was removed in JDK 9.

JEP 220 removed the well-known and frequently-referenced JARs rt.jar and tools.jar. According to it's description, "The class and resource files previously stored in lib/rt.jar, lib/tools.jar, lib/dt.jar, and various other internal jar files will now be stored in a more efficient format in implementation-specific files in the lib directory."