cmake HAS_ATTRIBUTE_WARN_UNUSED_RESULT examples

Lewuathe/mlir-hello thirdparty/llvm-project/polly/lib/External/CMakeLists.txt :74 

check_c_source_compiles("
  int func(void) __attribute__((__warn_unused_result__));
  int main(void) { return 0; }
  " HAS_ATTRIBUTE_WARN_UNUSED_RESULT)