AX_FIND_LIBRARY false negative (solves NETCONF support in FreeBSD and NETCONF unit tests in Alpine & CentOS)

The code was written to look for both lib and lib64 for libraries, but it always stops at lib if it couldn't find the library, so lib64 is forgotten due to bad logic in m4macro code. Affects NETCONF in FreeBSD.

Here is an ut-extended test where it complained about NETCONF. Notice how /usr/local/lib are left out (because it was found there), but it seems to not find the header and the library in all the other locations: https://jenkins.aws.isc.org/job/kea-dev/job/ut-extended/426/execution/node/366/log/

Here is a test run with the changes in the attached MR that shows it has passed the NETCONF check and now complains about Kerberos: https://jenkins.aws.isc.org/view/Kea-manual/job/kea-manual/job/ut-extended/56/flowGraphTable/