updated linux_app-20191223: x86_64-linux;
clarified location of lib/scripts/Isabelle.run;
Notes on building Poly/ML as Isabelle component
===============================================
* component skeleton:
$ isabelle build_polyml_component -s sha1 component
* include full source (without symlink), for example:
$ wget https://github.com/polyml/polyml/archive/master.zip