www/lighttpd: mod_authn_ldap.so linking to the wrong libldap.so

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

When building www/lighttpd with the ldap option, the resulting 
mod_authn_ldap.so links to the base libldap.so and not the pkgsrc one 
(despite databases/openldap-client/buildlink3.mk being .include'd).

On 6.1, this leads to a missing symbol (ldap_destroy), but it looks 
wrong anyway.

I guess that's some auto*/libtool/pkgconfig foo I don't understand.