Estou instalando a gem Ruby Nokogiri e encontrando o erro abaixo.

Como diagnosticar isso e resolvê-lo?

# gem install nokogiri
Building native extensions.  This could take a while...
ERROR:  Error installing nokogiri:
ERROR: Failed to build gem native extension.
...
/opt/ruby/1.9.3-p194/bin/ruby extconf.rb
checking for libxml/parser.h... *** 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.
...
/opt/ruby/1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:381:in `try_do': 
The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
from /opt/ruby/1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:506:in `try_cpp'
...