--- ruby-shadow-2.5.1/extconf.rb.orig	2022-11-30 15:03:24.946310154 +0900
+++ ruby-shadow-2.5.1/extconf.rb	2022-11-30 15:04:27.984276887 +0900
@@ -7,7 +7,7 @@
 require 'mkmf'
 require 'rbconfig'
 
-$CFLAGS = case RUBY_VERSION
+$CFLAGS = "#{$CFLAGS} " + case RUBY_VERSION
           when /^1\.9/; '-DRUBY19'
           when /^2\./; '-DRUBY19'
           when /^3\./; '-DRUBY19 -DRUBY30'