check_cxx_source_runs("
  extern \"C\" int folly_example_undefined_weak_symbol() __attribute__((weak));
  int main(int argc, char** argv) {
    auto f = folly_example_undefined_weak_symbol; // null pointer
    return f ? f() : 0; // must compile, link, and run with null pointer
  }"
  FOLLY_HAVE_WEAK_SYMBOLS
)