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

[Gregor Jasny]

On 15/11/15 21:13, digitalriptide wrote:
> Did this fix make it into 3.4? After upgrading to 3.4, GCC still seems
> to lack -isystem flags on OS X. Is there anything extra I need to do?

It should be fixed for the Makefile and Ninja generators but not Xcode.
My proposed fix broke older Xcode versions so that I had to revert it 
back. I did not find time to pick this up again.

Thanks,
Gregor