Direct users of IMPORTED targets treat INTERFACE_INCLUDE_DIRECTORIES
as SYSTEM, after commit a63fcbcb (Always consider includes from IMPORTED
targets to be SYSTEM., 2013-08-29). It was intended that transitive
use of an IMPORTED target would have the same behavior, but that
did not work. The implementation processed only direct dependencies
in cmTarget::FinalizeSystemIncludeDirectories.
Implement transitive evaluation of dependencies by traversing the
link interface of each target in the link implementation.
Unlike other target properties, this does not have a corresponding
non-INTERFACE variant.
This allows propagation of system attribute on include directories
from link dependents.
Stephen Kelly