Ensure that LINK_LIBRARIES can be populated without tll.

It should be allowed to use

 add_library(foo ...)
 add_library(bar ...)
 set_property(TARGET foo PROPERTY LINK_LIBRARIES bar)

without causing bar to be in the exported INTERFACE_LINK_LIBRARIES
of foo.
Merge topic 'handle-only-plain-tll' into next

42a17cc Ensure that LINK_LIBRARIES can be populated without tll.