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mac上安装rails的时候报错，
安装rails的在终端执行了一句命令：
$sudo gem install rails
报错了，报错信息为
clang: error: unknown argument: '-multiply_definedsuppress' [-Wunused-command-line-argument-hard-error-in-future]
完整的执行过程及报错信息如下：
angela@angeladeMacBook-Air:~$sudo gem install rails
Password:
Building native extensions. This could take a while...
ERROR: Error installing rails:
ERROR: Failed to build gem native extension.
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb
creating Makefile
make "DESTDIR="
compiling generator.c
linking shared-object json/ext/generator.bundle
clang: error: unknown argument: '-multiply_definedsuppress' [-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
make: *** [generator.bundle] Error 1
Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/json-1.8.1 for inspection.
Results logged to /Library/Ruby/Gems/2.0.0/gems/json-1.8.1/ext/json/ext/generator/gem_make.out
原因很简单，
Xcode 5.1中的苹果编译器将不认识的命令行视为错误行为。这类报错常出现在ruby和python的安装过程中，因为这两类程序中，很多编译语句较为特殊。
解决方案就是在安装语句前面加上这句
ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future ，
以安装rails为例,使用以下的语句：
$ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future gem install rails