https://git.ruby-lang.org/ruby.git/commit/?id=5ccf57f23e

From 5ccf57f23eb7a35ac699edc2fc598c47fa1844a0 Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@r...>
Date: Wed, 25 Mar 2020 22:00:15 +0900
Subject: Make RbConfig::CONFIG values mutable [Bug #16738]

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

(cherry picked from commit 4adb2d655dcd54b28366b4d17e86b21e2b622cd1)

diff --git a/test/test_rbconfig.rb b/test/test_rbconfig.rb
index 1bbf01b..fcbbbd8 100644
--- a/test/test_rbconfig.rb
+++ b/test/test_rbconfig.rb
@@ -51,4 +51,13 @@ class TestRbConfig < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/test_rbconfig.rb#L51
       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
diff --git a/tool/mkconfig.rb b/tool/mkconfig.rb
index 038fbf6..05ecb81 100755
--- a/tool/mkconfig.rb
+++ b/tool/mkconfig.rb
@@ -268,7 +268,7 @@ print <<EOS if $unicode_emoji_version https://github.com/ruby/ruby/blob/trunk/tool/mkconfig.rb#L268
   CONFIG["UNICODE_EMOJI_VERSION"] = #{$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)"
-- 
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