Add support for the jar 'm' (Manifest) option in UseJava.cmake

This is useful if one has to generate a .jar file that contains Class-Path: information for loading other .jar files at runtime. It's implemented in the same way the entrypoint option is.

Generating the Manifest file to use is probably best done by creating a template file and using file_configure(Manifest.in Manifest.out), depending on one's needs.