Un-break AIX build, take 2.

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