Makefile: revert libselinux: use ln --relative to create .so symlinks

This reverts 71393a181d63c9baae5fe8dcaeb9411d1f253998

ln --relative is too recent to be available in all distributions,
especially enterprise-grade distros that can stick around as long as
they are maintained (up to 10 years in some cases?).

For the sake of Buildroot, revert the upstream patch.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

diff -durNw libselinux-2.7.orig/src/Makefile libselinux-2.7/src/Makefile
--- libselinux-2.7.orig/src/Makefile	2018-01-15 20:53:50.168525700 +0100
+++ libselinux-2.7/src/Makefile	2018-01-15 20:55:27.061858005 +0100
@@ -181,7 +181,7 @@
 	install -m 755 $(LIBSO) $(LIBINSTALL)
 	test -d $(LIBINSTALL)/pkgconfig || install -m 755 -d $(LIBINSTALL)/pkgconfig
 	install -m 644 $(LIBPC) $(LIBINSTALL)/pkgconfig
-	ln -sf --relative $(LIBINSTALL)/$(LIBSO) $(LIBINSTALL)/$(TARGET)
+	cd $(LIBINSTALL) && ln -sf $(LIBSO) $(TARGET)
 
 install-pywrap: pywrap
 	test -d $(PYSITEDIR)/selinux || install -m 755 -d $(PYSITEDIR)/selinux