Fix autoconf check for mailspool world write permission

This is a bugfix and improvement for commit b15b7cc (``Use
realpath to find the mailspool'').

We can even do this check much simpler, if we focus on the
problem that

        $lspath -dlL $mailspool

does not check the target of a symlink, although -L is given.
The contradicting -d seems to overrule it.

We can fix this more easily by appending a slash to the path:

        $ ls -ld /var/mail/
        drwxrwxr-t 2 root root 4096 Apr 29 06:36 /var/mail//

Without the slash it was:

        $ ls -ld /var/mail
        lrwxrwxrwx 1 root root 10 Nov 22  2011 /var/mail -> spool/mail/

This small change does it for one level of indirection.
This should be sufficient.