Initializing it is the way to go, but it’s strange that it’s not erroring in mainline, we definitely have -Wall -Werror set. I wonder if this is occurring with LTO only, I don’t think we’re using it in official builds.
As for disabling warnings, I believe you can set a variable to inject the appropriate -Wno-whatever-warning flag, but I don’t recall what it is offhand. There’s a big block of comments at the top of the Makefile outlining configuration options.