[CMake] target_include_directories SYSTEM adds -isystem to Clang, but not GCC

[digitalriptide]

Dear CMake Community,

When I add SYSTEM to target_include_directories, for example using
target_include_directories( my_target SYSTEM PUBLIC ${MY_LIBRARIES} )
in Clang, CMake prepends -isystem to the relavent -I/my/library/path type
flags. With GCC, however, I see no -isystem added.

For reference, this is CMake 3.2.3 on OS X with Apple LLVM version 6.0 and
GCC 5 provided via MacPorts.

Should SYSTEM add -isystem with GCC?

Thank you for any tips!
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