Warning message for cmake code

When including a snippet of `CMakeLists.txt`, auto language discovery fails, so `lang:'cmake'` is necessary. But, when adding that, include-codeblock gives a warning:

```
warn: include-codeblock: unknown language `cmake`, use default
```

But the highlighting works fine.

Reproduce the warning by including a CMakeLists.txt snippet with lang:'cmake', then trace the include-codeblock language handling that reports cmake as unknown. Done means the warning is gone while CMake highlighting continues to work.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.