ando@sys-net.it writes:

It would be pretty straightforward to call lt_dlsetsearchpath() (actually, the module_path() wrapper) from module_init() with a default value. This could be, for example, LDAP_LIBEXECDIR, where slapd itself gets installed by default.

Use LDAP_LIBEXECDIR/openldap/, that's where modules get installed. Try with ./configure --enable-overlays=mod --enable-monitor=mod.