If the user specified --with-krb5 but we don't have a usable Kerberos

If the user specified --with-krb5 but we don't have a usable Kerberos
library, don't disable dissection and drive on, report an error and
stop; --with-krb5 means "build this with Kerberos", not "build this with
Kerberos if you can" (no -krb5 option at all means "build this with
Kerberos if you can").

If they *didn't* specify --with-krb5 and we don't have a usable Kerberos
library, set want_krb5 to "no", so that at the end of the configure
script we don't say we're building with the Kerberos library (otherwise,
want_krb5 is "ifpresent", but the stuff to print the configuration
options treats anything other than "no" as meaning "yes").

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@12118 f5534014-38df-0310-8fa8-9805f1628bb7