pip 错误unused-command-line-argument-hard-error-in-future解决办法

在我的mac air上，用pip安装一些python库时，偶尔就会遇到一些报错，关于“unused-command-line-argument-hard-error-in-future”，错误如下：

cc -fno-strict-aliasing -fno-common -dynamic -arch x86_64 -arch i386 -g -os -pipe -fno-common -fno-strict-aliasing -fwrapv -mno-fused-madd -denable_dtrace -dmacosx -dndebug -wall -wstrict-prototypes -wshorten-64-to-32 -dndebug -g -fwrapv -os -wall -wstrict-prototypes -denable_dtrace -arch x86_64 -arch i386 -pipe -i/system/library/frameworks/python.framework/versions/2.7/include/python2.7 -c psutil/_psutil_osx.c -o build/temp.macosx-10.9-intel-2.7/psutil/_psutil_osx.o

clang: error: unknown argument: '-mno-fused-madd' [-wunused-command-line-argument-hard-error-in-future]

clang: note: this will be a hard error (cannot be downgraded to a warning) in the future

error: command 'cc' failed with exit status 1

这样的错误，出现次数多了，每次都去google，不如自己记录一下吧。
原因是：the apple llvm compiler in xcode 5.1 treats unrecognized command-line options as errors. this issue has been seen when building both python native extensions and ruby gems, where some invalid compiler options are currently specified.

解决方法：设置archflags参数，如下:

sudo archflags=-wno-error=unused-command-line-argument-hard-error-in-future pip install psutil