Read -l flags from Makefiles for gems with native extensions

native_libs.rb

require 'set'
libs = Set.new

Gem::Specification.reject { |spec| spec.extensions.empty? }.each do |spec|
  spec.extensions.each do |ext_path|
    extconf_rb_path = Pathname.new(spec.gem_dir).join(ext_path)
    makefile_path = extconf_rb_path.dirname.join('Makefile')
    if makefile_path.readable?
      last_was_lib_flag = false
      makefile_path.readlines.map(&:strip).each do |line|
        next unless line.start_with?('LIBS =')
        line.gsub(/\s+/, ' ').sub('LIBS =', '').split.each do |lib|
          next if lib.start_with?('-L')
          lib = lib[2..-1] if lib.start_with?('-l')
          lib = Pathname.new(lib).basename.to_s.split('.').first if lib.include?('/')
          libs << lib
        end
      end
    end
  end
end

puts libs.to_a.sort.join("\n")