How to use the pythonnet.setupmono.PythonNET_BuildExt function in pythonnet

retcode = process.poll()
    if retcode:
        cmd = kwargs.get("args")
        if cmd is None:
            cmd = popenargs[0]
        raise CalledProcessError(retcode, cmd, output=output)
    return output


if __name__ == "__main__":
    setup(name="pythonnet",
          ext_modules=[
            Extension("clr", sources=[])
          ],
          cmdclass = {
            "build_ext" : PythonNET_BuildExt
          }

def build_extension(self, ext):
        """
        Builds the .pyd file using msbuild or xbuild.
        """
        if ext.name != "clr":
            return super(PythonNET_BuildExt, self).build_extension(ext)

        dest_file = self.get_ext_fullpath(ext.name)
        dest_dir = os.path.dirname(dest_file)
        if not os.path.exists(dest_dir):
            os.makedirs(dest_dir)

        defines = [
            "PYTHON%d%s" % (sys.version_info[:2]),
            "UCS2" if sys.maxunicode < 0x10FFFF else "UCS4",
        ]

        if CONFIG == "Debug":
            defines.extend(["DEBUG", "TRACE"])

        cmd = [
            _xbuild,