Be a little more explicit as to why we can't display a variable under watch.
Fix include paths when debugging a bytecode not at project root.
Do not allow an empty string to mean extra make options.

Make would complain that the target does not exist.
Fix empty string handling in makefile project properties.

All the options from the Makefile properties have an empty string meaning "nothing to add". However, current handling does not treat that specially, leading to malformed Make commands.
Make all the ocamldebug output parsing to be whitespace insensitive.

This should restore compatibility with older OCaml versions.