A few more fixes for missing links

Added vtksys where needed, changed the plugin code to use LINK_PUBLIC
(avoids problem of target_link_libraries changing behavior when using
the new signature on the same target as the old signature which
defaulted to public link interface libraries previously).

Change-Id: I3f20c48835191e4fb7e605f0aef959b3a66a99f6
Merge topic 'private-depends' into next

bb1c672 A few more fixes for missing links