A while back AC_PROG_LN_S was added to configure.ac and Makefile.am was converted to use $(LN_S) instead of ln -s, but there are 2 references that are propably overlooked:
$ grep ln Makefile.am
for i in $$what; do ln -s ../$$i . || exit $$? ; done )
ln -s $$1 $(DESTDIR)$$dir/$$2 && \
Done in http://cgit.freedesktop.org/systemd/systemd/commit/?id=ac2b34e15.