To test, create a new project with two files:

`main.cc`:

```cpp

int main(int argc, char **argv) { return 0; }
```

`CMakeLists.txt`:

```cmake
cmake_minimum_required(VERSION 3.29)

project(CppUnitsCompare)

include(FetchContent)
FetchContent_Declare(
  Au
  GIT_REPOSITORY file:///home/chogg/au
  GIT_TAG "chiphogg/cmake-install#215"
)
FetchContent_MakeAvailable(Au)

add_executable(CppUnitsCompare main.cpp)
target_link_libraries(CppUnitsCompare PUBLIC Au::au)
```

And execute:

```sh
cmake -S . -B build
cmake --build build
```

This PR is done when the above successfully compiles.

This appears to work, although I'm not yet sure whether it's a good
approach.

Not sure if the INCLUDEDIR needs to be updated too...