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