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.

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