This kinda feels like a repeat of https://bugzilla.redhat.com/show_bug.cgi?id=2215937.

Integrating iwyu into a CMake build system using CMAKE_C_INCLUDE_WHAT_YOU_USE and CMAKE_CXX_INCLUDE_WHAT_YOU_USE does not produce output.

The same steps work on Fedora 38.

There are 2 test files needed to reproduce this: `main.cpp` and `CMakeLists.txt`.

main.cpp:

```c++
#include <iostream>

#include <cstdio>

int main()
{
    std::cout << "Hello, World!" << std::endl;
    return 0;
}
```

CMakeLists.txt:

```cmake
cmake_minimum_required(VERSION 3.22)
project(iwyu_test)

set(CMAKE_CXX_STANDARD 17)

add_executable(iwyu_test main.cpp)
```

Reproducible: Always

Steps to Reproduce:
1. Fresh install Fedora 39 with the "C Development Tools and Libraries" group selected.
2. Update all packages: sudo dnf upgrade --refresh
3. Install required packages: sudo dnf install cmake iwyu
4. Put test files `main.cpp` and `CMakeLists.txt` into the same directory.
5. Configure CMake: cmake -B build -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE=include-what-you-use -DCMAKE_C_INCLUDE_WHAT_YOU_USE=include-what-you-use -DCMAKE_BUILD_TYPE=RelWithDebInfo
6. Start the build: cd build && cmake --build .
Actual Results:  
Build succeeds but there is no iwyu output.

Expected Results:  
```
Warning: include-what-you-use reported diagnostics:

/home/f38-test/iwyu-test/main.cpp should add these lines:

/home/f38-test/iwyu-test/main.cpp should remove these lines:
- #include <cstdio>  // lines 3-3

The full include-list for /home/f38-test/iwyu-test/main.cpp:
#include <iostream>  // for char_traits, operator<<, basic_ostream, endl, cout
---

```