buildsystem: Fix libfoo.so links for /lib libraries
* When installing to a non-default location by using DESTDIR on the make
installworld command line, /usr/lib/libfoo.so links pointed to wrong
locations
* DESTDIR is used both as an installworld parameter and internally by the
build system and these two cases expect different behaviors
* Symlinks locations in /usr/obj must be relative to the host (builder)
filesystem namespace
libc.so -> /usr/obj/usr/src/world_x86_64/lib/libc.so
* Symlinks locations in the final destination directory must be relative to
the target world filesystem namespace
libc.so -> /lib/libc.so
* The symlinks themselves must be absolute in order to not break
installations where /usr is a symlink
Inspired-by: FreeBSD's historical transition to /lib