Cannot install ruby gem 'libxml-ruby'

I am having trouble installing the ruby gem ‘libxml-ruby’. The output,
which I am struggling to understand, is below. It appears that I
require some ‘dependencies’, which is confusing because I thought
RubyGems was supposed to deal with dependencies. In addition, zlib is
part of the standard library so I don’t understand why I ‘need zlib’

Any assistance would be greatly appreciated.

gem install libxml-ruby
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 socket.lib… no
checking for gethostbyname() in nsl.lib… no
checking for atan() in m.lib… yes
checking for inflate() in z.lib… no
checking for inflate() in zlib1.lib… no
extconf failure: need zlib
*** 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:

Gem files will remain installed in
c:/ruby/lib/ruby/gems/1.8/gems/libxml-ruby- for inspection.
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