output

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

    current directory: /usr/local/lib/ruby/gems/2.4.0/gems/eventmachine-1.0.8/ext
/usr/local/bin/ruby -r ./siteconf20151227-28221-1s5nkv.rb extconf.rb
*** 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=/usr/local/bin/$(RUBY_BASE_NAME)
	--with-openssl-config
	--without-openssl-config
	--with-pkg-config
	--without-pkg-config
/usr/local/lib/ruby/2.4.0/shellwords.rb:81:in `shellsplit': undefined method `scan' for nil:NilClass (NoMethodError)
	from /usr/local/lib/ruby/2.4.0/mkmf.rb:1828:in `pkg_config'
	from extconf.rb:64:in `<main>'

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

  /usr/local/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-14/2.4.0-static/eventmachine-1.0.8/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /usr/local/lib/ruby/gems/2.4.0/gems/eventmachine-1.0.8 for inspection.
Results logged to /usr/local/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-14/2.4.0-static/eventmachine-1.0.8/gem_make.out