diff --git a/ext/dl/depend b/ext/dl/depend
index 50ef5cebbf..c66a7f0daf 100644
--- a/ext/dl/depend
+++ b/ext/dl/depend
@@ -1,4 +1,3 @@
-RUBY = $(RUBY_INSTALL_NAME)$(EXEEXT)
 CLEANFILES = test/test.o
 DISTCLEANFILES = call.func callback.func cbtable.func dlconfig.rb dlconfig.h \
                  test/libtest.so test/*~ *~ mkmf.log
@@ -42,4 +41,4 @@ cbtable.func: mkcbtable.rb dlconfig.rb
 	@$(RUBY) $< > $@
 
 debug:
-	$(MAKE) CFLAGS+=-DDEBUG
+	$(MAKE) CPPFLAGS="$(CPPFLAGS) -DDEBUG"