BaseTarget.xcconfig adds system paths to FRAMEWORK_SEARCH_PATHS instead of SYSTEM_FRAMEWORK_SEARCH_PATHS. This causes warnings (which become errors) to be emitted from system headers.
Happens in projects across the stack, including WebCore.
There’s also some use of -iframework and -isystem (not sure that one is still needed) in OTHER_CFLAGS that would be better expressed with SYSTEM_{FRAMEWORK,HEADERS}_SEARCH_PATHS