Experimental Heimdal Kerberos detection.

Experimental Heimdal Kerberos detection.
As this is experimental and does not yet work reliably this feature is
off by default. Enable it by using --with-krb5 during configure.
Please do not yet make use of this feature (i.e. please do not *use* the
defines provided by this option), until it has received further testing
and fixing.
Please run and check what the configure says.

svn path=/trunk/; revision=10395