ksu fails when -D flag is used and no DEBUG support was compiled.

* On Centos 6.x, ksu is compiled without debug support but when we try
to actually use the "-D" flag that is shown as a valid option, ksu
fails:

```
ccin2p3-vm # ksu targetuser -n remiferrand@IN2P3.FR -D
Usage: ksu [target user] [-n principal] [-c source cachename] [-k] [-D]
[-r time] [-pf] [-l lifetime] [-zZ] [-q] [-e command [args... ] ] [-a
[args... ] ]
ccin2p3-vm # echo $?
2
```

This patch makes ksu to accept `-D` as a valid option in `ksu` even if `DEBUG` was not compiled
in, but the option actually does nothing debug will not be activated.