<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/80117>80117</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            BUG: `openmp/cmake/HandleOpenMPOptions.cmake` uses CMake function `append` without defining it
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          h-vetinari
      </td>
    </tr>
</table>

<pre>
    While building openmp 18.1.0-rc1 on windows, I get:
```
CMake Error at cmake/HandleOpenMPOptions.cmake:47 (append):
  Unknown CMake command "append".
```

This is due to https://github.com/llvm/llvm-project/blame/release/18.x/openmp/cmake/HandleOpenMPOptions.cmake using `append` since https://github.com/llvm/llvm-project/commit/15fdc7646ca24506ccb6a59861da8f3b52ab031c, but not defining it (in contrast to `append_if` which explicitly duplicates the definition from [`HandleLLVMOptions.cmake`](https://github.com/llvm/llvm-project/blob/release/18.x/llvm/cmake/modules/HandleLLVMOptions.cmake):
https://github.com/llvm/llvm-project/blob/400a02bb281ba0de79211ab53d931e44dd76733f/openmp/cmake/HandleOpenMPOptions.cmake#L1-L10

I don't know why this file does not (or cannot) do `include(HandleLLVMOptions)`, but if that's not an option, then it needs to similarly vendor the definition of `append`.

CC @aganea 
</pre>
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