Error installing gem pg on Mac with Postgress.app

The pg gem will not install. Getting:

ERROR:  Error installing pg:
    ERROR: Failed to build gem native extension.

    current directory: /Library/Ruby/Gems/2.3.0/gems/pg-1.1.4/ext
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -r ./siteconf20191212-99334-1rync8p.rb extconf.rb
mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /Library/Ruby/Gems/2.3.0/gems/pg-1.1.4 for inspection.

I have tried
sudo gem install pg -- --with-pg-config=/Applications/Postgres.app/Contents/Versions/9.3/bin/pg_config
and
sudo ARCHFLAGS="-arch x86_64" gem install pg

and
bundle config build.pg --with-pg-config=/Applications/Postgres.app/Contents/Versions/9.3/bin/pg_config
and
sudo gem install pg -- --with-postgres-include-dir=/Applications/PostgresOld.app/Contents/Versions/9.3/include/ --with-postgres-lib-dir=/Applications/PostgresOld.app/Contents/Versions/9.3/lib/

All the same error

Is there a solution to this?