Add support for dependencies specified with a non-standard make target.

This is required for e.g. net/openntpd, which links against static libressl
libraries in that port's staging area.

In case of a port that is not fully installed, there is no clean phase for
the dependency's port directory. The dependency tracking does not maintain
the necessary state to perform this cleanup when the depending port is
finished.

PR: 213887
Reported by: blackmore@pichove.org (Simeon Simeonov)
Approved by: antoine (implicit)