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.