Not sure why exactly, but nokogiri installed fine on my desktop, but I've been 
fighting with the installation for hours on my MacBook Pro. When I did a search, 
it seems that this isn't limited to nokogiri, and can't seem to come up with a 
definitive fix. Here is what was logged in the gem_make.out file: 

/Users/wayneb/.rvm/rubies/ruby-1.9.2-p290/bin/ruby extconf.rb 
--with-xslt-dir=/usr/local/Cellar/libxslt/1.1.26
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.

<cut> 

/Users/wayneb/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:368:in 
`try_do': The complier failed to generate an executable file. (RuntimeError)
You have to install development tools first.
from /Users/wayneb/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:452:in 
`try_cpp'
from /Users/wayneb/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:853:in 
`block in find_header'
from /Users/wayneb/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:693:in 
`block in checking_for'
from /Users/wayneb/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:280:in 
`block (2 levels) in postpone'
from /Users/wayneb/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:254:in 
`open'
from /Users/wayneb/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:280:in 
`block in postpone'
from /Users/wayneb/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:254:in 
`open'
from /Users/wayneb/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:276:in 
`postpone'
from /Users/wayneb/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:692:in 
`checking_for'
from /Users/wayneb/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:852:in 
`find_header'
from extconf.rb:96:in `<main>'

I reinstalled the lasted Xcode "just in case"... that didn't fix anything. 

Anybody have a fix for this? It seems several gems have seen this issue, so I'm 
hoping by posting on the larger ruby list somebody else might have seen this and 
have a solution for it. 

Wayne