[CMake] does target_include_directories support SYSTEM?

[Matthew Woehlke]

See subject. I have a target that is the compiled sources from some 
stuff generated by Google Protobuf, which means using the headers also 
depend on ${PROTOBUF_INCLUDE_DIRS}. I would like to have this be a 
SYSTEM include... is this supported?

Also, is there a way to treat all interface include directories of 
imported targets as SYSTEM?

-- 
Matthew