Fix the load_* using test Makefiles not to link -ldl on NetBSD.
There is no such a library on NetBSD, and dlopen() is available
without a library. Quoting the manpage:
(These functions are not in a library. They are included in every
dynamically linked program automatically.)
To resolve this portably, introduce a new USE_LIBDL option. If it set
to 1, Makefile.rules automatically appends -ldl on platforms needing it.