BUILD: Fix cmocka detection
    
    We were not properly detecting that cmocka was unavailable. It was
    expecting an empty value and getting "no" instead. This patch
    corrects the expectation, so we will now skip building and running
    cmocka tests on platforms that do not have it available.
    
    Also, we were missing the cmocka header files in the distribution
    tarball, so 'make distcheck' was failing.