- Different libraries should have different EXPORTs, so that
you can IMPORT one while building the other. Reported (and
kindly explained) by Kevin Kofler.
- Stick to one header file, though.
While here, update copyright on file.
- Let's just have one header definining export- and visibility-
macros for Calamares. They are still selected based on the
export flags (*_PRO), just defined in one header instead of two.