title: gem install - fatal error: 'ruby/config.h' file not found in Mojave
body: <p>gem install is failing in MacOs Mojave. Anything that can help me solve this? My ruby version is <code>ruby 2.3.7p456</code>.      </p>
  
  <pre><code>➜ sudo gem install json -v '1.8.3'
  
  
  
  current directory: /Library/Ruby/Gems/2.3.0/gems/json-1.8.3/ext/json/ext/generator
  make "DESTDIR="
  compiling generator.c
  In file included from generator.c:1:
  In file included from ./../fbuffer/fbuffer.h:5:
  In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/include/ruby-2.3.0/ruby.h:33:
  /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/include/ruby-2.3.0/ruby/ruby.h:24:10: fatal error: 'ruby/config.h' file not found
  #include "ruby/config.h"
           ^~~~~~~~~~~~~~~
  1 error generated.
  make: *** [generator.o] Error 1
  
  make failed, exit code 2
  </code></pre>