4 files changed, 8 insertions, 4 deletions
diff --git a/ext/digest/md5/extconf.rb b/ext/digest/md5/extconf.rb
index 3fd21166a4..018f8ccb02 100644
--- a/ext/digest/md5/extconf.rb
+++ b/ext/digest/md5/extconf.rb
@@ -3,7 +3,8 @@
 
 require "mkmf"
 
-$CFLAGS << " -DHAVE_CONFIG_H -I#{File.dirname(__FILE__)}/.."
+$defs << "-DHAVE_CONFIG_H"
+$INCFLAGS << " -I$(srcdir)/.."
 
 $objs = [ "md5init.#{$OBJEXT}" ]
 
diff --git a/ext/digest/rmd160/extconf.rb b/ext/digest/rmd160/extconf.rb
index a3b63d75e0..d38dadd041 100644
--- a/ext/digest/rmd160/extconf.rb
+++ b/ext/digest/rmd160/extconf.rb
@@ -3,7 +3,8 @@
 
 require "mkmf"
 
-$CFLAGS << " -DHAVE_CONFIG_H -I#{File.dirname(__FILE__)}/.."
+$defs << "-DNDEBUG" << "-DHAVE_CONFIG_H"
+$INCFLAGS << " -I$(srcdir)/.."
 
 $objs = [ "rmd160init.#{$OBJEXT}" ]
 
diff --git a/ext/digest/sha1/extconf.rb b/ext/digest/sha1/extconf.rb
index 4d8ba64c39..d7b8126de5 100644
--- a/ext/digest/sha1/extconf.rb
+++ b/ext/digest/sha1/extconf.rb
@@ -3,7 +3,8 @@
 
 require "mkmf"
 
-$CFLAGS << " -DHAVE_CONFIG_H -I#{File.dirname(__FILE__)}/.."
+$defs << "-DHAVE_CONFIG_H"
+$INCFLAGS << " -I$(srcdir)/.."
 
 $objs = [ "sha1init.#{$OBJEXT}" ]
 
diff --git a/ext/digest/sha2/extconf.rb b/ext/digest/sha2/extconf.rb
index 894228deca..200d386bef 100644
--- a/ext/digest/sha2/extconf.rb
+++ b/ext/digest/sha2/extconf.rb
@@ -3,7 +3,8 @@
 
 require "mkmf"
 
-$CPPFLAGS << " -DHAVE_CONFIG_H -I#{File.dirname(__FILE__)}/.."
+$defs << "-DHAVE_CONFIG_H"
+$INCFLAGS << " -I$(srcdir)/.."
 
 $objs = [
   "sha2.#{$OBJEXT}",