libevent sync
Author: Trond Norbye <trond.norbye@gmail.com>
Date:   Wed Feb 19 06:49:31 2014 +0100

    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.