M_SQRT1_2未在OpenCL中定义

Question

我试图在Xcode中编译以下内容：

#pragma OPENCL EXTENSION cl_khr_fp64 : enable

double normcdf(double x) {
    return 0.5 * erfc(-x * M_SQRT1_2);
}

但我收到错误：error: use of undeclared identifier 'M_SQRT1_2'

如何使用常量M_SQRT1_2？