mechanizeのupdateではまってる

rubygemsのmechanizeをupdateしようとして失敗しているのだけど，どうしたらいいかさっぱり．

$ sudo gem update mechanize
Updating installed gems
Updating mechanize
Building native extensions.  This could take a while...
ERROR:  While executing gem ... (Gem::Installer::ExtensionBuildError)
    ERROR: Failed to build gem native extension.

/usr/local/bin/ruby18 extconf.rb update mechanize
checking for libxml/parser.h in /opt/local/include/,/opt/local/include/libxml2,/usr/local/include/libxml2,/usr/local/include,/usr/local/include/libxml2,/usr/include/libxml2... no
libxml2 is missing.  try 'port install libxml2' or 'yum install libxml2'

/usr/ports/textproc/libxml2は入ってるんだけどなぁ．