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.