Un-break AIX build, take 2.
I incorrectly diagnosed the reason why hoverfly is unhappy.
Looking closer, it appears that it fails to link libldap
unless libssl is also present; so the problem was my
idea of clearing LIBS before making the check. Revert
to essentially the original coding, except that instead
of failing when libldap_r isn't there, use libldap.
Per buildfarm member hoverfly.
Discussion: https://postgr.es/m/17083-
a19190d9591946a7@postgresql.org