Make RbConfig::CONFIG values mutable [Bug #16738]

As `RbConfig.expand` modifies the argument and involved `CONFIG`
values, its values should be mutable.

test/test_rbconfig.rb

@@ -51,4 +51,13 @@ class TestRbConfig < Test::Unit::TestCase
       assert_match(/\$\(sitearch|\$\(rubysitearchprefix\)/, val, "#{key} #{bug7823}")
     end
   end
+
+  if /darwin/ =~ RUBY_PLATFORM
+    def test_sdkroot
+      assert_separately([{"SDKROOT" => "$(prefix)/SDKRoot"}], "#{<<~"begin;"}\n#{<<~'end;'}")
+      begin;
+        assert_equal RbConfig::CONFIG["prefix"]+"/SDKRoot", RbConfig::CONFIG["SDKROOT"]
+      end;
+    end
+  end
 end

tool/mkconfig.rb

@@ -268,7 +268,7 @@ print <<EOS if $unicode_emoji_version
   CONFIG["UNICODE_EMOJI_VERSION"] = #{$unicode_emoji_version.dump}
 EOS
 print <<EOS if /darwin/ =~ arch
-  CONFIG["SDKROOT"] = ENV["SDKROOT"] || "" # don't run xcrun every time, usually useless.
+  CONFIG["SDKROOT"] = "\#{ENV['SDKROOT']}" # don't run xcrun every time, usually useless.
 EOS
 print <<EOS
   CONFIG["archdir"] = "$(rubyarchdir)"