sudo gem install knife-ec2

gistfile1.txt

Building native extensions.  This could take a while...
ERROR:  Error installing knife-ec2:
	ERROR: Failed to build gem native extension.

    /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby -r ./siteconf20150119-12518-1qmgu0.rb extconf.rb
checking if the C compiler accepts ... yes
checking if the C compiler accepts -Wno-error=unused-command-line-argument-hard-error-in-future... no
Building nokogiri using packaged libraries.
checking for iconv... yes
************************************************************************
IMPORTANT NOTICE:

Buidling Nokogiri with a packaged version of libxml2-2.9.2
with the following patches applied:
	- 0001-Revert-Missing-initialization-for-the-catalog-module.patch
	- 0002-Fix-missing-entities-after-CVE-2014-3660-fix.patch

Team Nokogiri will keep on doing their best to provide security
updates in a timely manner, but if this is a concern for you and want
to use the system library instead; abort this installation process and
reinstall nokogiri as follows:

    gem install nokogiri -- --use-system-libraries
        [--with-xml2-config=/path/to/xml2-config]
        [--with-xslt-config=/path/to/xslt-config]

If you are using Bundler, tell it to use the option:

    bundle config build.nokogiri --use-system-libraries
    bundle install

Note, however, that nokogiri is not fully compatible with arbitrary
versions of libxml2 provided by OS/package vendors.
************************************************************************
Extracting libxml2-2.9.2.tar.gz into tmp/x86_64-apple-darwin14/ports/libxml2/2.9.2... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxml2/0001-Revert-Missing-initialization-for-the-catalog-module.patch...
Running 'patch' for libxml2 2.9.2... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxml2/0002-Fix-missing-entities-after-CVE-2014-3660-fix.patch...
Running 'patch' for libxml2 2.9.2... OK
Running 'configure' for libxml2 2.9.2... OK
Running 'compile' for libxml2 2.9.2... OK
Running 'install' for libxml2 2.9.2... OK
Activating libxml2 2.9.2 (from /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/x86_64-apple-darwin14/libxml2/2.9.2)...
************************************************************************
IMPORTANT NOTICE:

Buidling Nokogiri with a packaged version of libxslt-1.1.28
with the following patches applied:
	- 0001-Adding-doc-update-related-to-1.1.28.patch
	- 0002-Fix-a-couple-of-places-where-f-printf-parameters-wer.patch
	- 0003-Initialize-pseudo-random-number-generator-with-curre.patch
	- 0004-EXSLT-function-str-replace-is-broken-as-is.patch
	- 0006-Fix-str-padding-to-work-with-UTF-8-strings.patch
	- 0007-Separate-function-for-predicate-matching-in-patterns.patch
	- 0008-Fix-direct-pattern-matching.patch
	- 0009-Fix-certain-patterns-with-predicates.patch
	- 0010-Fix-handling-of-UTF-8-strings-in-EXSLT-crypto-module.patch
	- 0013-Memory-leak-in-xsltCompileIdKeyPattern-error-path.patch
	- 0014-Fix-for-bug-436589.patch
	- 0015-Fix-mkdir-for-mingw.patch

Team Nokogiri will keep on doing their best to provide security
updates in a timely manner, but if this is a concern for you and want
to use the system library instead; abort this installation process and
reinstall nokogiri as follows:

    gem install nokogiri -- --use-system-libraries
        [--with-xml2-config=/path/to/xml2-config]
        [--with-xslt-config=/path/to/xslt-config]

If you are using Bundler, tell it to use the option:

    bundle config build.nokogiri --use-system-libraries
    bundle install
************************************************************************
Extracting libxslt-1.1.28.tar.gz into tmp/x86_64-apple-darwin14/ports/libxslt/1.1.28... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxslt/0001-Adding-doc-update-related-to-1.1.28.patch...
Running 'patch' for libxslt 1.1.28... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxslt/0002-Fix-a-couple-of-places-where-f-printf-parameters-wer.patch...
Running 'patch' for libxslt 1.1.28... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxslt/0003-Initialize-pseudo-random-number-generator-with-curre.patch...
Running 'patch' for libxslt 1.1.28... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxslt/0004-EXSLT-function-str-replace-is-broken-as-is.patch...
Running 'patch' for libxslt 1.1.28... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxslt/0006-Fix-str-padding-to-work-with-UTF-8-strings.patch...
Running 'patch' for libxslt 1.1.28... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxslt/0007-Separate-function-for-predicate-matching-in-patterns.patch...
Running 'patch' for libxslt 1.1.28... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxslt/0008-Fix-direct-pattern-matching.patch...
Running 'patch' for libxslt 1.1.28... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxslt/0009-Fix-certain-patterns-with-predicates.patch...
Running 'patch' for libxslt 1.1.28... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxslt/0010-Fix-handling-of-UTF-8-strings-in-EXSLT-crypto-module.patch...
Running 'patch' for libxslt 1.1.28... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxslt/0013-Memory-leak-in-xsltCompileIdKeyPattern-error-path.patch...
Running 'patch' for libxslt 1.1.28... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxslt/0014-Fix-for-bug-436589.patch...
Running 'patch' for libxslt 1.1.28... OK
Running patch with /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/patches/libxslt/0015-Fix-mkdir-for-mingw.patch...
Running 'patch' for libxslt 1.1.28... OK
Running 'configure' for libxslt 1.1.28... OK
Running 'compile' for libxslt 1.1.28... OK
Running 'install' for libxslt 1.1.28... OK
Activating libxslt 1.1.28 (from /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5/ports/x86_64-apple-darwin14/libxslt/1.1.28)...
checking for main() in -llzma... yes
checking for xmlParseDoc() in libxml/parser.h... no
checking for xmlParseDoc() in -lxml2... no
checking for xmlParseDoc() in -llibxml2... no
-----
libxml2 is missing.  Please locate mkmf.log to investigate how it is failing.
-----
*** 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/2.0/usr/bin/ruby
	--help
	--clean
	--use-system-libraries
	--enable-static
	--disable-static
	--with-zlib-dir
	--without-zlib-dir
	--with-zlib-include
	--without-zlib-include=${zlib-dir}/include
	--with-zlib-lib
	--without-zlib-lib=${zlib-dir}/lib
	--enable-cross-build
	--disable-cross-build
	--with-xml2lib
	--without-xml2lib
	--with-libxml2lib
	--without-libxml2lib

extconf failed, exit code 1

Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/nokogiri-1.6.5 for inspection.
Results logged to /Library/Ruby/Gems/2.0.0/extensions/universal-darwin-14/2.0.0/nokogiri-1.6.5/gem_make.out