Whilst trying to install the Fog gem on OS X Mountain Lion recently I kept experiencing issues with libxml.
The error output was this:
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.
So this was ruby-1.9.3-p327.
After trying numerous things to get this to play ball (switches passed to gem, Homebrew installed libxslt and libxml) I decided to build the latest 1.9.3 available in RVM, -p448.
The problem went away.
So this must be something to do with the way the OS X RVM ruby-1.9.3p327 binary is compiled.