错误信息：

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

    /usr/bin/ruby2.1 -r ./siteconf20150328-1540-hff2f0.rb extconf.rb
checking if the C compiler accepts ... *** 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=/usr/bin/ruby2.1
  --help
  --clean
/usr/lib/ruby/2.1.0/mkmf.rb:456:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
  from /usr/lib/ruby/2.1.0/mkmf.rb:571:in `block in try_compile'
  from /usr/lib/ruby/2.1.0/mkmf.rb:522:in `with_werror'
  from /usr/lib/ruby/2.1.0/mkmf.rb:571:in `try_compile'
  from extconf.rb:80:in `nokogiri_try_compile'
  from extconf.rb:87:in `block in add_cflags'
  from /usr/lib/ruby/2.1.0/mkmf.rb:621:in `with_cflags'
  from extconf.rb:86:in `add_cflags'
  from extconf.rb:337:in `<main>'

extconf failed, exit code 1

Gem files will remain installed in /var/lib/gems/2.1.0/gems/nokogiri-1.6.6.2 for inspection.
Results logged to /var/lib/gems/2.1.0/extensions/x86_64-linux/2.1.0/nokogiri-1.6.6.2/gem_make.out

配置：

1. Ubuntu 14.10（GNU / Linux 3.16.0-23-通用x86_64）
2. gcc（Ubuntu 4.9.1-16ubuntu6）4.9.1
3. 红宝石2.1.2p95（2014-05-08）[x86_64-linux-gnu]