Add -Qunused-arguments for clang on macos The clang compiler provided with macosx emits warnings like: CC bufferevent.lo clang: warning: argument unused during compilation: '-I .' clang: warning: argument unused during compilation: '-I ./compat' clang: warning: argument unused during compilation: '-I ./include' clang: warning: argument unused during compilation: '-I ./include' for each file being compiled. This generates a lot of noise during compilation making it hard to see "real" errors. This patch mute those warnings.