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.

Reviewed-on: http://gerrit.openafs.org/2739
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 696e1c015eeef259451c101813b9e930e402cb49)

Change-Id: Ia69e1899f1e3d058271f147d33b58383ca35b4ec
Reviewed-on: http://gerrit.openafs.org/3985
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>