Compiler warnings can't be disabled when using FetchContent

[nilsvu]

I followed the instructions in the README to include yaml-cpp in my CMake project with FetchContent_Declare. When building the code, I get compiler warnings like those listed in #1208. This is not unexpected. However, I want to silence these compiler warnings since I can't do anything about them and they alert the users of my code. Since CMake version 3.25 this is possible with the SYSTEM option passed to FetchContent_Declare: https://cmake.org/cmake/help/latest/module/FetchContent.html#command:fetchcontent_declare
This option works for other external packages that I fetch, but it has no effect on yaml-cpp. I still get the compiler warnings even if I use the SYSTEM option. Can this be fixed?

[jbeder]

Not sure how - open to suggestions?