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$ gem install idn-ruby -v '0.1.0' --source 'https://rubygems.org/'
Building native extensions. This could take a while...
ERROR:  Error installing idn-ruby:
	ERROR: Failed to build gem native extension.

    current directory: /Users/hyejinan/.rvm/gems/ruby-2.6.0/gems/idn-ruby-0.1.0/ext
/Users/hyejinan/.rvm/rubies/ruby-2.6.0/bin/ruby -I /Users/hyejinan/.rvm/rubies/ruby-2.6.0/lib/ruby/2.6.0 -r ./siteconf20190521-27028-kds8w9.rb extconf.rb
checking for -lidn... no
ERROR: could not find idn library!

  Please install the GNU IDN library or alternatively specify at least one
  of the following options if the library can only be found in a non-standard
  location:
    --with-idn-dir=/path/to/non/standard/location
        or
    --with-idn-lib=/path/to/non/standard/location/lib
    --with-idn-include=/path/to/non/standard/location/include

*** 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
	--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=/Users/hyejinan/.rvm/rubies/ruby-2.6.0/bin/$(RUBY_BASE_NAME)
	--with-idn-dir
	--without-idn-dir
	--with-idn-include
	--without-idn-include=${idn-dir}/include
	--with-idn-lib
	--without-idn-lib=${idn-dir}/lib
	--with-idnlib
	--without-idnlib

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /Users/hyejinan/.rvm/gems/ruby-2.6.0/extensions/x86_64-darwin-17/2.6.0/idn-ruby-0.1.0/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/hyejinan/.rvm/gems/ruby-2.6.0/gems/idn-ruby-0.1.0 for inspection.
Results logged to /Users/hyejinan/.rvm/gems/ruby-2.6.0/extensions/x86_64-darwin-17/2.6.0/idn-ruby-0.1.0/gem_make.out