I am compiling on OSX 10.11 with clang-3.7 from macports. Here's some sample code to demonstrate the problem:

    #include <stdio.h>
    #include <stdlib.h>

    int main() {
    #if(__has_builtin(__builtin_cpu_supports)) && defined(__x86_64__)
      printf("__builtin_cpu_supports: %d\n", __builtin_cpu_supports("sse3"));
    #else
      printf("__builtin_cpu_cupports not available\n");
    #endif
      return EXIT_SUCCESS;
    }

If I compile with clang-3.7 or clang-3.8 (both which claim to have the builtin cpu supports), everything works. However, during linking, I get the following:

    Undefined symbols for architecture x86_64:
      "___cpu_model", referenced from:
          _dt_init in darktable.c.o
    ld: symbol(s) not found for architecture x86_64

With earlier versions of clang, I see the else clause.

I have also seen this in the wild where this problem exists for FreeBSD as well. https://github.com/darktable-org/darktable/commit/081bc5823870918160fdd93d0157d56263add7d5.