这个是我在SO上的提问，可以去看一下。
过程：
sudo gem install eventmachine
...
Building native extensions. This could take a while...
/Users/kuankuan/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/rubygems/ext/builder.rb:48: warning: Insecure world writable dir /usr/local/bin/g4bin in PATH, mode 040777
ERROR: Error installing eventmachine:
ERROR: Failed to build gem native extension.
/Users/kuankuan/.rvm/rubies/ruby-1.9.3-p327/bin/ruby extconf.rb
/Users/kuankuan/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/mkmf.rb:1344: warning: Insecure world writable dir /usr/local/bin/g4bin in PATH, mode 040777
checking for rb_trap_immediate in ruby.h,rubysig.h... no
checking for rb_thread_blocking_region()... yes
checking for inotify_init() in sys/inotify.h... no
checking for __NR_inotify_init in sys/syscall.h... no
checking for writev() in sys/uio.h... yes
checking for rb_thread_check_ints()... yes
checking for rb_time_new()... yes
checking for sys/event.h... yes
checking for sys/queue.h... yes
creating Makefile
make
compiling binder.cpp
cc1plus: error: unrecognized command line option "-arch"
make: *** [binder.o] Error 1
ruby版本：1.9.3 Mac OSX 10.8.3
汗，我和你同样的机器同样的配置一条命令就1秒钟就装好了。这个好难回答。你的xcode版本多少，commandline升级了没？一般mac下ruby环境出错会和这个有关。。