how can i build static program by cmake+clang+nmake on windows?

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The following is perfectly operational but require, at least, CMake version 3.15 (you specified version 3.5 through cmake_minimum_required() command):

cmake_minimum_required(VERSION 3.15)

set(CMAKE_C_COMPILER "C:/Software/LLVM/bin/clang.exe")
set(CMAKE_CXX_COMPILER "C:/Software/LLVM/bin/clang++.exe")

project(helloworld)

set(CMAKE_VERBOSE_MAKEFILE ON)

set(CMAKE_CXX_STANDARD 17)
set(SOURCE helloworld.cpp)
add_executable(${PROJECT_NAME} ${SOURCE})
set_property(TARGET main PROPERTY MSVC_RUNTIME_LIBRARY MultiThreadedDebug)

By the way, I strongly suggest to you to read the CMake documentation. For example, setting variable CMAKE_CXX_COMPILER_TARGET outside a toolchain file is irrelevant.