Support for the Ruby 2.4 series has ended. See here for reference.
# File rubygems/ext/cmake_builder.rb, line 5 def self.build(extension, directory, dest_path, results, args=[], lib_dir=nil) unless File.exist?('Makefile') then cmd = "cmake . -DCMAKE_INSTALL_PREFIX=#{dest_path}" cmd << " #{Gem::Command.build_args.join ' '}" unless Gem::Command.build_args.empty? run cmd, results end make dest_path, results results end