Don't error out when recreating a symlink in $(bindir)

"ln -s" fails if the target file already exists.  however, $(LN_S) can be expanded to "ln" or "cp" for portability reasons, though unlikely on recent platforms:
https://gnu.org/software/autoconf/manual/autoconf.html#AC_005fPROG_005fLN_005fS
how about explicitly using "ln -sf" here?