Summary
- updated to cygwin-20190320;
- updated to Cygwin 3.0.4 (approximation for Isabelle2019);
- prefer ISABELLE_OCAML_SETUP: Cygwin lacks libzarith;
- more robust reference to ghc exe (with multi-platform support);
- more robust: allow empty root (e.g. via symlink);
- avoid prompt;
- updated settings: ISABELLE_OCAMLFIND, ISABELLE_OCAML_SETUP, but retain compatibility with historic versions that require ISABELLE_OCAMLC;
- access OCaml tools and libraries via ISABELLE_OCAMLFIND; OPAM setup is optional: it requires odd development tools that are not available in default OS installations (e.g. make, m4);
- proper ISABELLE_HOME (ISABELLE_ROOT is for platform-specific application bootstrap);
- more robust: allow empty root (e.g. via symlink);