diff --git a/ext/etc/extconf.rb b/ext/etc/extconf.rb
index 1e8bd16d1d..2e28d58037 100644
--- a/ext/etc/extconf.rb
+++ b/ext/etc/extconf.rb
@@ -44,14 +44,16 @@ have_struct_member('struct group', 'gr_passwd', 'grp.h')
 # for https://github.com/ruby/etc
 srcdir = File.expand_path("..", __FILE__)
 constdefs = "#{srcdir}/constdefs.h"
-ruby = RbConfig.ruby
-if File.file?(ruby)
-  ruby = [ruby]
-else
-  require "shellwords"
-  ruby = Shellwords.split(ruby)
+if !File.exist?(constdefs)
+  ruby = RbConfig.ruby
+  if File.file?(ruby)
+    ruby = [ruby]
+  else
+    require "shellwords"
+    ruby = Shellwords.split(ruby)
+  end
+  system(*ruby, "#{srcdir}/mkconstants.rb", "-o", constdefs)
 end
-system(*ruby, "#{srcdir}/mkconstants.rb", "-o", constdefs)
 
 # TODO: remove when dropping 2.7 support, as exported since 3.0
 have_func('rb_deprecate_constant(Qnil, "None")')