Makefile.sub: refine distclean-ext

* win32/Makefile.sub (distclean-ext, realclean-ext): remove
  extinit.c and suppress error message when failed to remove ext
  directory.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57394 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

win32/Makefile.sub

@@ -1072,7 +1072,8 @@ clean-ext distclean-ext realclean-ext::
 		) )
 
 distclean-ext realclean-ext::
-		-$(Q)rmdir ext
+		-$(Q)$(RM) ext/extinit.c
+		-$(Q)rmdir ext 2> nul || @
 
 clean-extout: