bus: use ln -fs to enable dbus in systemd, not $(LN_S)

bus: use ln -fs to enable dbus in systemd, not $(LN_S)

Using $(LN_S) is inappropriate because it could in theory mean either
ln -s, ln or cp -p depending on autoconf checks.

Not using -f breaks reinstallation directly from source (DESTDIR unset),
because the symlinks will already exist.

Because systemd isn't currently portable to non-Linux, let alone
non-SUS-compliant systems, it seems safe to assume that ln -fs behaves
as specified by SUS if systemd was found.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=37870
Reviewed-by: Colin Walters <walters@verbum.org>