Xcode: Obey SYSTEM keyword for includes (#15687)
CMake used to put all header search paths into HEADER_SEARCH_PATHS attribute. Unfortunately this attribute does not support to declare a search path as a system include. As a hack one could add a -isystem /path to the cflags but then include ordering is not deterministic. A better approach was chosen with this patch by not filling HEADER_SEARCH_PATHS at all and to populate the C, C++, and Fortran flags directly. The include paths used by Xcode should be now identical to the ones used by Unix Makefiles and Ninja generator.