software/lib: make install should unconditionally create the soft link to the library
Currently, the software/lib Makefile checks if libmockturtle.so is present in /usr/local/lib and, if not, it creates it.
This is not correct behaviour, since once a new version of the lib is out, the lib will be installed but the link will still point to the older version.
test -e $(DESTLIBDIR)/$(LIBS) should be removed and the soft-link creation should be forced (
ln -sf), to ensure that the link is always up-to-date.