Fixes https://github.com/harfbuzz/harfbuzz/issues/864

Unfortunately 1.7.7 and 1.8.0 went out with the wrong hb-version.h contents.

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+.

HB_VERSION_CHECK's comparison was originally written wrongly
by mistake.  When API tests were written, they were also written
wrongly to pass given the wrong implementation... Sigh.

Given the purpose of this API, there's no point in fixing it
without renaming it.  As such, rename.

API changes:

  HB_VERSION_CHECK -> HB_VERSION_ATLEAST
  hb_version_check -> hb_version_atleast

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.