Since commit v3.0.0-rc1~174^2~1 (cmTarget: Fix system include annotation
propagation, 2014-01-01) the cmGeneratorTarget::IsSystemIncludeDirectory
method needs to collect all targets that might provide
INTERFACE_(|SYSTEM)_INCLUDE_DIRECTORIES for the current target.  We now
have cmTarget::GetLinkImplementationClosure to provide this, so use it.