libsepol: rework host installation

libsepol: rework host installation

libsepol has a pretty peculiar interpretation of DESTDIR and PREFIX.
PREFIX is not consistently used: some installation paths are forced to
$(DESTDIR)/usr/... . Also, the shared lib symlink is forced to ../../lib
(i.e. assumes it's installed under /usr). For these reasons, the host
build and install commands are a bit unusual: they use DESTDIR instead
of PREFIX, DESTDIR points to $(HOST_DIR) without /usr, and some
additional fixup is needed to correct the installation directory of
libsepol.so.1 and the symlink to it.

This can be simplified quite a lot by passing both DESTDIR and PREFIX,
and including the /usr part in both. The symlink still has to be fixed,
but that's much simpler.

Note that a side effect is that the man pages are now installed under
$(HOST_DIR)/usr/usr/man. While not very nice, we don't really care
about man pages. In addition, this will resolve itself when we later
move $(HOST_DIR)/usr to $(HOST_DIR).

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