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.

從連結的 gist 中的最小可重現 CMakeLists.txt 開始，檢查 FetchContent 設定，然後重現產生錯誤。檢查預期如何讓 dawn 函式庫到達 third_party/lib 以及 _deps/gpucpp-{build/subbuild/src} 路徑。完成的標準是：在不手動放置 libdawn.dylib 的情況下，乾淨的 CMake 建置能夠找到該函式庫和標頭檔。

由索引模型根據 Issue 內容生成。