LIBADD: Try to support partial tree checkouts in some limited cases.

LIBADD is only supported for in-tree builds because we do not install
share/mk/src.libnames.mk (which provides LIBADD support) into /usr/share/mk.
So if a partial checkout is done then the LIBADDs are ignored and no LDADD is
ever added.

Provide limited support for this case for when LIBADD is composed entirely of
base libraries. This is to avoid clashes with ports and other out-of-tree
LIBADD uses that should not be mapped to LDADD and because we do not want to
support LIBADD out-of-tree right now.

Reported by: mckusick, kib
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon