LU-3490 autoconf: disable GSS when required libraries missing

LU-3490 autoconf: disable GSS when required libraries missing

Add 'auto' functionality to GSS keyring configuration to match
GSS configuration.

When using the 'auto' setting for GSS and GSS keyring, set
enable_gss/enable_gss_keyring to 'yes' if configuration tests pass
so GSS is actually built.

Disable GSS/KRB5 if both krb5_derive_key and krb5int_derive_key
are missing. In some distro such as Sles11sp1 and Ubuntu 10.04,
both of these libraries are missing

Remove #ifdef HAVE_STRUCT_CRED since it's not defined anywhere.
This is due to incomplete cleanup of autoconf from LU-2800

Signed-off-by: Patrick Farrell <paf@cray.com>
Signed-off-by: Minh Diep <minh.diep@intel.com>
Change-Id: I36eb1370afe42cbac3ac924ca8100acaa25558d9
Reviewed-on: http://review.whamcloud.com/7622
Tested-by: Hudson
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>