<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/62366>62366</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Default build of libcxx fails with 'Unknown CMake command CHECK_C_SOURCE_COMPILES'
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          rnikander
      </td>
    </tr>
</table>

<pre>
    I filed a similar bug about the [default build instructions](https://libcxx.llvm.org/BuildingLibcxx.html) not working: https://github.com/llvm/llvm-project/issues/61488 

But since this is a new error, so I thought I'd create a new issue. 

I followed the instructions exactly, and got this:

```console
% cmake -G Ninja -S runtimes -B build -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind"
[...]
CMake Error at /Users/rob/Dev/llvm-project/llvm/cmake/modules/HandleLLVMOptions.cmake:809 (CHECK_C_SOURCE_COMPILES):
  Unknown CMake command "CHECK_C_SOURCE_COMPILES".
Call Stack (most recent call first):
 CMakeLists.txt:159 (include)
```

Using CMake 3.26. 
</pre>
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