Ruby extensions

Building ruby gems with C extensions.

Documentation

ruby extension docs	https://github.com/ruby/ruby/blob/master/doc/extension.rdoc
gems with extensions docs	https://guides.rubygems.org/gems-with-extensions/

Ruby Cli Params

INCLUDE/LIB directories (extconf.rb params).

# https://bugs.ruby-lang.org/issues/7173
ruby \
  --with-opt-dir /opt/package  # add dir with 'include/' and 'lib/' subdirs (c headers/libs)