libafsauthent: Build with libtool

libafsauthent: Build with libtool

Construct the libafsauthent library with libtool. All libraries
contributing to libafsauthent provide a libauthent_<library>.la
convenience library, which we use libtool to combine into shared
and static versions of libafsauthent.

Change-Id: I76fa61eb027862eab730c4c86ab74a7b7e03c13b
Reviewed-on: http://gerrit.openafs.org/8097
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Derrick Brashear <shadow@your-file-system.com>