Add support for GSSAPIDelegateCredentials in ssh_config.

By default this only applies to Solaris 11.
This is not correct. 
The default GSSAPIDelegateCredentials on Solaris 11 (like all other Solaris ) is No.

Please see the below section from the man pages.

GSSAPIDelegateCredentials

     Enables/disables  GSS-API  credential  forwarding.   The
     default is no.

The best way will be to simply define the variable and leave it undef