在mac下安装的，按照教程：https://ruby-china.org/wiki/install_ruby_guide

版本：ruby 2.0.0p59

　　　rvm 1.26.9 (latest)

　　　gem 2.4.5

报错：

AppleXiao:~ AppleXiao$ gem install rails
Building native extensions. This could take a while...
ERROR: Error installing rails:
ERROR: Failed to build gem native extension.

/Users/AppleXiao/.rvm/rubies/ruby-2.0.0-p598/bin/ruby -r ./siteconf20150118-59594-tbfkfm.rb extconf.rb
checking if the C compiler accepts ... yes
checking if the C compiler accepts -Wno-error=unused-command-line-argument-hard-error-in-future... no
Building nokogiri using packaged libraries.
-----
The file "/usr/include/iconv.h" is missing in your build environment,
which means you haven't installed Xcode Command Line Tools properly.

To install Command Line Tools, try running `xcode-select --install` on
terminal and follow the instructions. If it fails, open Xcode.app,
select from the menu "Xcode" - "Open Developer Tool" - "More Developer
Tools" to open the developer site, download the installer for your OS
version and run it.
-----
*** 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
--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=/Users/AppleXiao/.rvm/rubies/ruby-2.0.0-p598/bin/ruby
--help
--clean
--use-system-libraries
--enable-static
--disable-static
--with-zlib-dir
--without-zlib-dir
--with-zlib-include
--without-zlib-include=${zlib-dir}/include
--with-zlib-lib
--without-zlib-lib=${zlib-dir}/lib
--enable-cross-build
--disable-cross-build

extconf failed, exit code 1

Gem files will remain installed in /Users/AppleXiao/.rvm/gems/ruby-2.0.0-p598/gems/nokogiri-1.6.5 for inspection.
Results logged to /Users/AppleXiao/.rvm/gems/ruby-2.0.0-p598/extensions/x86_64-darwin-14/2.0.0/nokogiri-1.6.5/gem_make.out

查了很多资料，不知道问题在哪里，请指教，谢谢！