Only build and run cmocka unit tests if its submodule is initialized

Only build and run cmocka unit tests if its submodule is initialized

Commit 40cb4cfc5d01110 added infrastructure to write unit tests using
cmocka.  This was implemented using a git submodule to fetch an
up-to-date cmocka test framework.

The issue which appeared was that 'make check' stopped working if
the cmocka submodule was not initialized and updated.  As we do not
want this to be a hard depenency, this patch makes running these
unit tests conditional.  If cmocka has not been initialized, skip
them or if it has been initialized all unit tests will be run.

 [v2 - Also check if cmake is available, as cmocka depends on that
       to be built ]

Signed-off-by: David Sommerseth <dazo@privateinternetaccess.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1464703645-26640-1-git-send-email-openvpn@sf.lists.topphemmelig.net>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11758
(cherry picked from commit 45f6e7991cfa3bb8a44f981b6cf1e794d617d51e)