SystemModeler 4.3 is quite old and the support libraries that simulations are linked with was at that time built with libstdc++, so you cannot change to link with libc++ (that's why you get link errors).
What you need is an Xcode old enough to ship with libstdc++ and my googling tells me that Xcode 9 or older should do that. I verified on my machine that 4.3 works with Xcode 9.2 with the default compiler flags (-m64 -stdlib=libstdc++ -fPIC).
Otto