Had some issues installing Nokogiri this morning on my slicehost account running Ubuntu (Hardy 8.04). Every time I tried installing the gem, I would get this :

sudo gem install nokogiri
Building native extensions.  This could take a while...
ERROR:  Error installing nokogiri:
	ERROR: Failed to build gem native extension.

/usr/bin/ruby1.8 extconf.rb install nokogiri
checking for #include

... no
need libxml
*** 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=/usr/bin/ruby1.8

Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/nokogiri-1.1.1 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/nokogiri-1.1.1/ext/nokogiri/gem_make.out

Solution?

Not sure really, I did all of these, but it wasnt’ until the last one that the gem successfully installed.

sudo aptitude install libxml
sudo aptitude install libxml2
sudo aptitude install libxml-dev
sudo aptitude install libxml2-dev
sudo aptitude install libxslt
sudo aptitude install libxslt-dev

And that’s it. After that, the gem installed just fine.