libselinux: rework host installation

libselinux has a pretty peculiar interpretation of DESTDIR and PREFIX.
PREFIX is not consistently used: some installation paths are forced to
$(DESTDIR)/usr/... . In other cases, PREFIX is indeed used. PREFIX
defaults to $(DESTDIR)/usr.

Try to be a little bit more correct by passing both DESTDIR and PREFIX,
both set to $(HOST_DIR). This is not a complete fix: man pages are
still installed in $(HOST_DIR)/usr - but we don't care about that.

Also simplify the symlink creation, like how it's done in libsepol.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>