解决国内无法使用gem

     安装 idb tool。

    今天使用gem命令，出现了下面的错误：

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

    current directory: /usr/local/lib/ruby/gems/2.4.0/gems/libxml-ruby-3.0.0/ext/libxml
/usr/local/opt/ruby/bin/ruby -r ./siteconf20170221-5409-1vufyx7.rb extconf.rb
/usr/local/opt/ruby/bin/ruby: warning: shebang line ends with \r may cause a problem
checking for libxml/xmlversion.h in /opt/include/libxml2,/opt/local/include/libxml2,/usr/local/include/libxml2,/usr/include/libxml2... no
*** 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=/usr/local/Cellar/ruby/2.4.0/bin/$(RUBY_BASE_NAME)
        --with-xml2-config
        --without-xml2-config
        --with-xml2-dir
        --without-xml2-dir
        --with-xml2-include
        --without-xml2-include=${xml2-dir}/include
        --with-xml2-lib
        --without-xml2-lib=${xml2-dir}/lib
 extconf failure: need libxml2.

    Install the library or try one of the following options to extconf.rb:

      --with-xml2-config=/path/to/xml2-config
      --with-xml2-dir=/path/to/libxml2
      --with-xml2-lib=/path/to/libxml2/lib
      --with-xml2-include=/path/to/libxml2/include


To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /usr/local/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-15/2.4.0/libxml-ruby-3.0.0/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /usr/local/lib/ruby/gems/2.4.0/gems/libxml-ruby-3.0.0 for inspection.
Results logged to /usr/local/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-15/2.4.0/libxml-ruby-3.0.0/gem_make.out

     想了很久，Google 很久，还是没得进展。

     －－－－－－－－－替换gem 默认地址－－－－－－－－－－

     会想起上个os 版本，我安装swift的相关东西，需要将gem的sources更换。 

     国内 访问：https://rubygems.org/  有限制。

    执行下面的命令： 删除原有sources 源。

＊＊deMacBook-Air:~ $ gem sources --remove https://rubygems.org/
https://rubygems.org/ removed from sources
＊＊deMacBook-Air:~ $ gem sources -a https://ruby.taobao.org/
https://ruby.taobao.org/ added to sources

      再执行安装。搞定：