gem install nokogiri -v '1.5.4'
Building native extensions. This could take a while...
ERROR: Error installing nokogiri:
ERROR: Failed to build gem native extension.

/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/mkmf.rb:997: warning: Insecure world writable dir /usr/local in PATH, mode 040777
checking for libxml/parser.h... yes
checking for libxslt/xslt.h... yes
checking for libexslt/exslt.h... yes
checking for iconv_open() in iconv.h... no
checking for iconv_open() in -liconv... no
checking for iconv_open() in -liconv... no
checking for libiconv_open() in iconv.h... no
checking for libiconv_open() in -liconv... no
checking for libiconv_open() in -liconv... no
-----
libiconv is missing. please visit http://nokogiri.org/tutorials/installing_nokogiri.html for help with installing dependencies.
-----
*** 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=/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
--with-zlib-dir
--without-zlib-dir
--with-zlib-include
--without-zlib-include=${zlib-dir}/include
--with-zlib-lib
--without-zlib-lib=${zlib-dir}/lib
--with-iconv-dir
--without-iconv-dir
--with-iconv-include
--without-iconv-include=${iconv-dir}/include
--with-iconv-lib
--without-iconv-lib=${iconv-dir}/lib
--with-xml2-dir
--without-xml2-dir
--with-xml2-include
--without-xml2-include=${xml2-dir}/include
--with-xml2-lib
--without-xml2-lib=${xml2-dir}/lib
--with-xslt-dir
--without-xslt-dir
--with-xslt-include
--without-xslt-include=${xslt-dir}/include
--with-xslt-lib
--without-xslt-lib=${xslt-dir}/lib
--with-libxslt-config
--without-libxslt-config
--with-pkg-config
--without-pkg-config
--with-libxml-2.0-config
--without-libxml-2.0-config
--with-libiconv-config
--without-libiconv-config
--with-iconvlib
--without-iconvlib
--with-iconvlib
--without-iconvlib
--with-iconvlib
--without-iconvlib
--with-iconvlib
--without-iconvlib

extconf failed, exit code 1