/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby -rubygems /Library/Ruby/Gems/2.0.0/gems/rake-0.9.6/bin/rake RUBYARCHDIR=/Library/Ruby/Gems/2.0.0/extensions/universal-darwin-14/2.0.0/xcodeproj-0.17.0 RUBYLIBDIR=/Library/Ruby/Gems/2.0.0/extensions/universal-darwin-14/2.0.0/xcodeproj-0.17.0
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb
checking for -std=c99 option to compiler... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
    --with-opt-dir
    --without-opt-dir
    --with-opt-include
    --without-opt-include=${opt-dir}/include
    --with-opt-lib
    --without-opt-lib=${opt-dir}/lib
    --with-make-prog
    --without-make-prog
    --srcdir=.
    --curdir
    --ruby=/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:434:in `try_do‘: The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:549:in `block in try_compile‘
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:502:in `with_werror‘
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:549:in `try_compile‘
    from extconf.rb:24:in `block in <main>‘
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:895:in `block in checking_for‘
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:340:in `block (2 levels) in postpone‘
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:310:in `open‘
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:340:in `block in postpone‘
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:310:in `open‘
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:336:in `postpone‘
    from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/mkmf.rb:894:in `checking_for‘
    from extconf.rb:23:in `<main>‘
rake aborted!
Command failed with status (1): [/System/Library/Frameworks/Ruby.framework/...]
/Library/Ruby/Gems/2.0.0/gems/xcodeproj-0.17.0/ext/xcodeproj/Rakefile:37:in `block in <top (required)>‘
Tasks: TOP => default => ext
(See full trace by running task with --trace)

首先安装Xcode6 的 Command Line Tools，在开发者中心下载

接下来按照下面步骤操作：

打开 Xcode 6
打开 Preferences
选择 Locations 选项卡
将 Command Line Tools 选择为 Xcode 6.0
卸载 cocoapods
$ sudo gem uninstall cocoapods
安装 xcodeproj
$ sudo gem install xcodeproj
再安装 cocoapods
$ sudo gem install cocoapods
最后运行 pod --version 就看到Pod正常了。