Refactor: Use Submodule for efsw Instead of Manual Source Files

[LianYangCn]

The integration of efsw has been updated to use a git submodule rather than directly including individual source files. This makes future updates cleaner and more maintainable.

It’s now linked as a static library via add_subdirectory, and a few minor tweaks were made externally to improve portability without modifying the original source. As a bonus, this also helps to reduce overall build time.

把 efsw 的引用方式改成了 git submodule 的方式，这样以后维护更简单的一些。

efsw 本身支持 cmake, 所以直接在顶层 CMakeLists.cmake 中用 add_subdirectory 来引用更加可维护，只要原作者的代码是可靠的，这里就是可靠的。

另外小改了一下顶层 CMakeLists.cmake 让其更加模块化