aklog: Fix weak_crypto tests

aklog: Fix weak_crypto tests

The tests for the various ways of enabling weak cryptography fail
with current Heimdal master, because it defines krb5_allow_weak_crypto
but does not prototype it.

Fix this by testing for the Heimdal version (which MIT does not provide)
first, and only if that's not available, try to use allow_weak_crypto.

Change-Id: I559d5fd40c196fefc947dd0f7b10ed78fbd2c7e6
Reviewed-on: http://gerrit.openafs.org/2739
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>