The libatf-* major version numbers have differed from upstream,
which is just confusing and bound to cause problems down the road.
This change reverts the major version numbers to 0. This should be OK
to do because the libraries are now private and they are only installed
when WITH_TESTS is enabled, which is not yet the default. However, we
have to tell users to start with a clean object tree to ensure the test
programs get rebuilt against the new numbered libraries.