New API:
- hb_set_previous()
- hb_set_previous_range()

This prepares the headers for exporting symbols using visibility
attributes or __declspec(dllexport), so that we do not need to maintain
symbols listing files, as this is what was and is done in GLib and GTK+.

Fixes https://github.com/behdad/harfbuzz/issues/103

Fails now.  Fixing.

TODO: Tests for hb_set_t.

Users should #include <hb.h> (or hb-ft.h, hb-glib.h, etc), but
never things like hb-shape.h directly.  This makes it easier to
refactor headers later on without breaking compatibility.

Step version up to 0.5.0.

Also, fix to pass "make distcheck"

Also optimizes the common-direction script lookup.

This reverts commit c939f6aff405ca7b10b1f1538f46148bff719fcb.