Add user-friendly error to configure script when running in improper location

Here’s a small patch to add a user-friendly error message.

I.e. if you run “cd auto; ./configure” instead of “./auto/configure”,
you get:

./configure: 10: .: Can’t open auto/options

With this patch you now get:

No ‘auto’ subdir found, are you running this from the root dir?

Patch attached, please submit to svn.
Thanks