[Feature #20345] Add `--target-rbconfig` option to mkmf

Introduce a new mkmf option `--target-rbconfig` to specify the RbConfig
file for the deployment target platform. This option is useful for
cross-compiling Ruby extensions without faking the global top-level
`RbConfig` constant.

lib/mkmf.rb

@@ -44,6 +44,23 @@ end
 # correctly compile and link the C extension to Ruby and a third-party
 # library.
 module MakeMakefile
+
+  target_rbconfig = nil
+  ARGV.delete_if do |arg|
+    opt = arg.delete_prefix("--target-rbconfig=")
+    unless opt == arg
+      target_rbconfig = opt
+    end
+  end
+  if target_rbconfig
+    # Load the RbConfig for the target platform into this module.
+    # Cross-compiling needs the same version of Ruby.
+    Kernel.load target_rbconfig, self
+  else
+    # The RbConfig for the target platform where the built extension runs.
+    RbConfig = ::RbConfig
+  end
+
   #### defer until this module become global-state free.
   # def self.extended(obj)
   #   obj.init_mkmf

test/mkmf/test_config.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: false
 require_relative 'base'
+require 'tempfile'
 
 class TestMkmfConfig < TestMkmf
   def test_dir_config
@@ -27,4 +28,30 @@ class TestMkmfConfig < TestMkmf
       assert_equal(false, with_config("foo"))
     end;
   end
+
+  def test_with_target_rbconfig
+    Tempfile.create(%w"rbconfig .rb", ".") do |tmp|
+      tmp.puts <<~'end;'
+      module RbConfig
+        CONFIG = {}
+        MAKEFILE_CONFIG = {}
+
+        def self.fire_update!(key, value); end
+        def self.expand(val, config = CONFIG); val; end
+      end;
+      ::RbConfig::CONFIG.each do |k, v|
+        tmp.puts "  CONFIG[#{k.dump}] = #{v.dump}"
+      end
+      ::RbConfig::MAKEFILE_CONFIG.each do |k, v|
+        tmp.puts "  MAKEFILE_CONFIG[#{k.dump}] = #{v.dump}"
+      end
+      tmp.puts "  CONFIG['testing-only'] = 'ok'"
+      tmp.puts "end"
+      tmp.close
+      assert_separately([], ["--target-rbconfig=#{tmp.path}"], <<-'end;')
+        assert_equal("ok", MakeMakefile::RbConfig::CONFIG["testing-only"])
+        assert_not_equal(::RbConfig, MakeMakefile::RbConfig)
+      end;
+    end
+  end
 end