Add GSS-API (from MIT Kerberos 5) in configure
Some details:
- the new argument will be --with-gssapi pointing to the krb5-config script i.e. the same as for bind9
- the implementation must be the MIT one (vs Heimdal)
- this will be in the core configure in a new m4 macro file
- of course the default will be to not configure with GSS-API support