libxml-ruby-2.3.2 fails to build on Ubuntu 11.04 with jRuby-1.6.6

libxml-ruby build on linux

jruby-1.6.6 15:02 ~/.rvm/gems/jruby-1.6.6/gems/libxml-ruby-2.3.2/ext/libxml $ ruby extconf.rb 
WARNING: JRuby does not support native extensions or the `mkmf' library very well.
Check http://kenai.com/projects/jruby/pages/Home for alternatives.
checking for socket() in -lsocket... no
checking for gethostbyname() in -lnsl... yes
checking for atan() in -lm... no
checking for atan() in -lm... yes
checking for inflate() in -lz... yes
checking for iconv_open() in -liconv... no
checking for libiconv_open() in -liconv... no
checking for libiconv_open() in -llibiconv... no
checking for iconv_open() in -llibiconv... no
checking for iconv_open() in -lc... yes
checking for xmlParseDoc() in -lxml2... yes
checking for libxml/xmlversion.h... no
checking for libxml/xmlversion.h in /opt/include/libxml2,/usr/local/include/libxml2,/usr/include/libxml2... yes
creating extconf.h
creating Makefile


jruby-1.6.6 15:02 ~/.rvm/gems/jruby-1.6.6/gems/libxml-ruby-2.3.2/ext/libxml $ make
cc -I. -I. -I/home/david/.rvm/rubies/jruby-1.6.6/lib/native/include/ruby -I. -I/usr/include/libxml2 -I/usr/local/include -DRUBY_EXTCONF_H=\"extconf.h\"        -fPIC  -fno-omit-frame-pointer -fno-strict-aliasing  -fexceptions   -I. -I/home/david/.rvm/rubies/jruby-1.6.6/lib/native/include/ruby -I. -I/usr/include/libxml2 -I/usr/local/include  -m64 -c libxml.c
libxml.c:6:18: fatal error: util.h: No such file or directory
compilation terminated.
make: *** [libxml.o] Error 1

jruby-1.6.6 15:02 ~/.rvm/gems/jruby-1.6.6/gems/libxml-ruby-2.3.2/ext/libxml $