Unable to build with cmake FetchContent

Expected: Use FetchContent_Delcare to declare the dependency and then use FetchContent_MakeAvailable and the built library and headers will be in the _deps/gpucpp-{build/subbuild/src}

Actual: The generating process ends up with an error. Error message and minimal reproducible CMakeLists.txt is inside the [gist](https://gist.github.com/kagurazaka-ayano/64a806bff6de0c6a7ba7a00d12eff3dc)

The dawn library file doesn't appear in the third_party/lib directory. After I built [this](https://github.com/jspanchu/webgpu-dawn-binaries) and put the libdawn.dylib into the third_party/lib directory the cmake command works.