$ sudo gem install libxml-ruby

Select which gem to install for your platform (i486-linux)
 1. libxml-ruby 0.9.0 (ruby)
 2. libxml-ruby 0.9.0 (x86-mswin32-60)
 3. libxml-ruby 0.8.3 (ruby)
 4. libxml-ruby 0.8.3 (x86-mswin32-60)
 5. Skip this gem
 6. Cancel installation
> 1
Building native extensions.  This could take a while...
ERROR:  While executing gem ... (Gem::Installer::ExtensionBuildError)
    ERROR: Failed to build gem native extension.

ruby extconf.rb install libxml-ruby
checking for socket() in -lsocket... no
checking for gethostbyname() in -lnsl... yes
checking for atan() in -lm... no
checking for atan() in -lm... yes
checking for inflate() in -lz... yes
checking for iconv_open() in -liconv... no
checking for libiconv_open() in -liconv... no
checking for libiconv_open() in -llibiconv... no
checking for iconv_open() in -llibiconv... no
checking for iconv_open() in -lc... yes
checking for xmlParseDoc() in -lxml2... yes
checking for libxml/xmlversion.h... no
checking for #include <libxml/xmlversion.h>
... yes
checking for xmlDocFormatDump()... yes
checking for docbCreateFileParserCtxt()... yes
creating extconf.h
creating Makefile

make
cc -I. -I. -I/usr/lib/ruby/1.8/i486-linux -I. -I/usr/include/libxml2 -I/usr/local/include -DRUBY_EXTCONF_H=\"extconf.h\"  -fPIC -fno-strict-aliasing -g -g -O2  -fPIC -I. -I/usr/lib/ruby/1.8/i486-linux -I. -I/usr/include/libxml2 -I/usr/local/include  -c ruby_xml_xinclude.c
cc -I. -I. -I/usr/lib/ruby/1.8/i486-linux -I. -I/usr/include/libxml2 -I/usr/local/include -DRUBY_EXTCONF_H=\"extconf.h\"  -fPIC -fno-strict-aliasing -g -g -O2  -fPIC -I. -I/usr/lib/ruby/1.8/i486-linux -I. -I/usr/include/libxml2 -I/usr/local/include  -c ruby_xml_html_parser.c
cc -I. -I. -I/usr/lib/ruby/1.8/i486-linux -I. -I/usr/include/libxml2 -I/usr/local/include -DRUBY_EXTCONF_H=\"extconf.h\"  -fPIC -fno-strict-aliasing -g -g -O2  -fPIC -I. -I/usr/lib/ruby/1.8/i486-linux -I. -I/usr/include/libxml2 -I/usr/local/include  -c ruby_xml_relaxng.c
cc -I. -I. -I/usr/lib/ruby/1.8/i486-linux -I. -I/usr/include/libxml2 -I/usr/local/include -DRUBY_EXTCONF_H=\"extconf.h\"  -fPIC -fno-strict-aliasing -g -g -O2  -fPIC -I. -I/usr/lib/ruby/1.8/i486-linux -I. -I/usr/include/libxml2 -I/usr/local/include  -c ruby_xml_error.c
ruby_xml_error.c: In function âruby_init_xml_errorâ:
ruby_xml_error.c:207: error: âXML_FROM_SCHEMATRONVâ undeclared (first use in this function)
ruby_xml_error.c:207: error: (Each undeclared identifier is reported only once
ruby_xml_error.c:207: error: for each function it appears in.)
ruby_xml_error.c:514: error: âXML_TREE_NOT_UTF8â undeclared (first use in this function)
ruby_xml_error.c:890: error: âXML_SCHEMATRONV_ASSERTâ undeclared (first use in this function)
ruby_xml_error.c:891: error: âXML_SCHEMATRONV_REPORTâ undeclared (first use in this function)
make: *** [ruby_xml_error.o] Error 1


Gem files will remain installed in /var/lib/gems/1.8/gems/libxml-ruby-0.9.0 for inspection.
Results logged to /var/lib/gems/1.8/gems/libxml-ruby-0.9.0/ext/libxml/gem_make.out