HA hook depends on http library which is not linked with servers.
So kea-dhcp4 from the build directory (vs installed) fails to load the HA hook. The solution is to add the http library in the dhcp4 and dhcp6 Makefile.am files. It adds a dependency which is not used in the common case, at the other hand it is not good to dynamic load a Kea standard library with a hook.