krb5 1.2.3 compile errors

[Ken Raeburn]

> we should probably modify configure to look for xlc first, gcc second,
> and then cc last.

We aren't currently using a search list.  If you set $CC or specify
--with-cc=FOO, it uses it; otherwise, we use "cc".  Maybe we should
change it, but I'd recommend just going with the standard autoconf
macros, and whatever they look for....  (I.e., unless there's a good
reason why krb5 should be different from all other autoconf packages,
fix any problems at the autoconf level.)