diff --git a/ext/digest/bubblebabble/extconf.rb b/ext/digest/bubblebabble/extconf.rb
index 21feed57fb..8fb001c61d 100644
--- a/ext/digest/bubblebabble/extconf.rb
+++ b/ext/digest/bubblebabble/extconf.rb
@@ -1,6 +1,4 @@
 # frozen_string_literal: false
 require 'mkmf'
 
-$defs << "-DHAVE_CONFIG_H"
-
 create_makefile('digest/bubblebabble')
diff --git a/ext/digest/md5/extconf.rb b/ext/digest/md5/extconf.rb
index dead9a228b..01658e1b60 100644
--- a/ext/digest/md5/extconf.rb
+++ b/ext/digest/md5/extconf.rb
@@ -6,8 +6,6 @@
 require "mkmf"
 require File.expand_path("../../digest_conf", __FILE__)
 
-$defs << "-DHAVE_CONFIG_H"
-
 $objs = [ "md5init.#{$OBJEXT}" ]
 
 digest_conf("md5")
diff --git a/ext/digest/rmd160/extconf.rb b/ext/digest/rmd160/extconf.rb
index a923dcf9be..8b894da0c8 100644
--- a/ext/digest/rmd160/extconf.rb
+++ b/ext/digest/rmd160/extconf.rb
@@ -9,7 +9,6 @@ require File.expand_path("../../digest_conf", __FILE__)
 if try_static_assert("RUBY_API_VERSION_MAJOR < 3", "ruby/version.h")
   $defs << "-DNDEBUG"
 end
-$defs << "-DHAVE_CONFIG_H"
 
 $objs = [ "rmd160init.#{$OBJEXT}" ]
 
diff --git a/ext/digest/sha1/extconf.rb b/ext/digest/sha1/extconf.rb
index 1e94ba3ef9..7582cd9ab5 100644
--- a/ext/digest/sha1/extconf.rb
+++ b/ext/digest/sha1/extconf.rb
@@ -6,8 +6,6 @@
 require "mkmf"
 require File.expand_path("../../digest_conf", __FILE__)
 
-$defs << "-DHAVE_CONFIG_H"
-
 $objs = [ "sha1init.#{$OBJEXT}" ]
 
 digest_conf("sha1")
diff --git a/ext/digest/sha2/extconf.rb b/ext/digest/sha2/extconf.rb
index 8bf28111b6..3b772675e1 100644
--- a/ext/digest/sha2/extconf.rb
+++ b/ext/digest/sha2/extconf.rb
@@ -6,8 +6,6 @@
 require "mkmf"
 require File.expand_path("../../digest_conf", __FILE__)
 
-$defs << "-DHAVE_CONFIG_H"
-
 $objs = [ "sha2init.#{$OBJEXT}" ]
 
 unless digest_conf("sha2")