I'm sorry, my patch doesn't work on Xcode (Apple LLVM).
computed-gotos is still enabled by default.

Apple doesn't expose LLVM version.  It's really annoying.

$ cat x.c
#include <stdio.h>

int main()
{
    printf("__clang__ : %d\n", __clang__);
    printf("__llvm__ : %d\n", __llvm__);
    printf("__VERSION__ : %s\n", __VERSION__);
    printf("__clang_version__ : %s\n", __clang_version__);
    printf("__clang_major__   : %d\n", __clang_major__);
    printf("__clang_minor__   : %d\n", __clang_minor__);
    printf("__clang_patchlevel__ : %d\n", __clang_patchlevel__);
}

$ cc x.c && ./a.out
__clang__ : 1
__llvm__ : 1
__VERSION__ : 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.39.2)
__clang_version__ : 9.0.0 (clang-900.0.39.2)
__clang_major__   : 9
__clang_minor__   : 0
__clang_patchlevel__ : 0