[PATCH] Trivial fix to dep.rtlinux script

I wanted to get comedi for rtlinux 3.2-pre2 to compile correctly.  I
actually found that in the current cvs version of comedi it works without
a hitch!

At any rate while trying to get it to compile I accidentally mistyped my
rtl directory and comedi's ./configure script didn't complain at all.
While this isn't a showstopper (as the astute user will figure this out
eventually), a minor usability fix was to have the dep.rtlinux script in
the scripts/ subdirectory figure out that the directory specified may not
be valid and complain to the user about this.  This saves a few headaches
for developers that are trying to get comedi working with rtlinux and may
encounter this problem (as I did).

Attached is the small patch to make the dep.rtlinux scripts slightly more
intelligent with respect to the RTLDIR...

This is against the current cvs version of comedi.. dave, is this ok with
you?

-Calin