Watir问题---gem install watir-classic报错： ERROR: Error installing watir-classic: ERROR: Failed to build gem native extension.

Watir在安装watir-classic的时候报错，如下：

c:devkit>gem install watir-classic  --platform=mswin32
Temporarily enhancing PATH to include DevKit...
Building native extensions.  This could take a while...
ERROR:  Error installing watir-classic:
        ERROR: Failed to build gem native extension.

        C:/Ruby193/bin/ruby.exe extconf.rb
checking for libxml/parser.h... no
-----
libxml2 is missing.  please visit http://nokogiri.org/tutorials/installing_nokogiri.html for help wi
th installing dependencies.
-----
*** 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=C:/Ruby193/bin/ruby
        --with-zlib-dir
        --without-zlib-dir
        --with-zlib-include
        --without-zlib-include=${zlib-dir}/include
        --with-zlib-lib
        --without-zlib-lib=${zlib-dir}/lib
        --with-iconv-dir
        --without-iconv-dir
        --with-iconv-include
        --without-iconv-include=${iconv-dir}/include
        --with-iconv-lib
        --without-iconv-lib=${iconv-dir}/lib
        --with-xml2-dir
        --without-xml2-dir
        --with-xml2-include
        --without-xml2-include=${xml2-dir}/include
        --with-xml2-lib
        --without-xml2-lib=${xml2-dir}/lib
        --with-xslt-dir
        --without-xslt-dir
        --with-xslt-include
        --without-xslt-include=${xslt-dir}/include
        --with-xslt-lib
        --without-xslt-lib=${xslt-dir}/lib
        --with-libxslt-config
        --without-libxslt-config
        --with-pkg-config
        --without-pkg-config
        --with-libxml-2.0-config
        --without-libxml-2.0-config
        --with-pkg-config
        --without-pkg-config
        --with-libiconv-config
        --without-libiconv-config
        --with-pkg-config
        --without-pkg-config


Gem files will remain installed in C:/Ruby193/lib/ruby/gems/1.9.1/gems/nokogiri-1.6.0.rc1 for inspec
tion.
Results logged to C:/Ruby193/lib/ruby/gems/1.9.1/gems/nokogiri-1.6.0.rc1/ext/nokogiri/gem_make.out

解决办法参考文章：http://stackoverflow.com/questions/1948043/ruby-cannot-install-watir-gem-on-windows?rq=1

I also had to install nokogiri to get Watir to install successfully on windows, after installing DevKit. Use: gem install nokogiri

即在安装好Devkit后，需要安装nokogiri，安装成功后，才能继续安装Watir，不然会报错