This tracking was added during the development of commit 042ecf04
(Add API to calculate link-interface-dependent bool properties
or error., 2013-01-06), but was never used.

It was not necessary to use the content because what is really
useful in that logic is to determine if a property has been implied
to be null by appearing in a LINK_LIBRARIES genex.

I think the motivating usecase for developing the feature of
keeping track of the targets relevant to a property was that I
thought it would  make it possible to allow requiring granular
compatibility of interface properties only for targets which
depended on the interface property. Eg:

 add_library(foo ...)
 add_library(bar ...)

 add_executable(user ...)
 # Read the INTERFACE_POSITION_INDEPENDENT_CODE from bar, but not
 # from foo:
 target_link_libraries(user foo $<$<TARGET_PROPERTY:POSTITION_INDEPENDENT_CODE>:bar>)

This obviously doesn't make sense. We require that INTERFACE
properties are consistent across all linked targets instead.