Fix creating install directories in `make sharedinstall` if they exist
already outside `DESTDIR`.  The previous make rules assumed that
the directories would be created via a dependency on a rule
for `$(DESTSHARED)` that did not fire if the directory did exist outside
`$(DESTDIR)`.

While technically `$(DESTDIR)` could be prepended to the rule name,
moving the rules for creating directories straight into
the `sharedinstall` rule seems to fit the common practices better.
Since the rule explicitly checks whether the individual directories
exist anyway, there seems to be no reason to rely on make determining
that implicitly as well.