Unix builds: Reference krb5 libs in standard way

Unix builds: Reference krb5 libs in standard way

Use $(LIB_krb5) and $(LDFLAGS_krb5) to reference the Kerberos
library and linker flags, rather than directly using an autoconf
substitution. This brings us in line with the way other libraries
are handled.

Change-Id: Icda16833abea32440a5c307c39eb06b32613b06a
Reviewed-on: http://gerrit.openafs.org/7583
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>