nokogiri -> ERROR: cannot discover where libxml2 is located on your system

nokogiri_install

# `ERROR:  Error installing nokogiri:
# ERROR: Failed to build gem native extension.
# 
#   current directory: /usr/local/var/rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/nokogiri-1.7.0/ext/nokogiri
# /usr/local/var/rbenv/versions/2.3.1/bin/ruby -r ./siteconf20170103-68488-r71c9j.rb extconf.rb --with-xml=/usr/local/Cellar/libxml2/ --use-system-libraries
# 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 system libraries.
# ERROR: cannot discover where libxml2 is located on your system. please make sure `pkg-config` is installed.
# *** 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.`

gem install nokogiri -- \ 
  --with-xml2-include=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/libxml2 \
  --use-system-libraries

# copy/paste one-liner
gem install nokogiri -- --with-xml2-include=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/libxml2 --use-system-libraries