Don't use bundled zlib.

$OpenBSD: patch-custom_Makefile_in,v 1.1.1.1 2009/04/24 13:13:45 wcmaier Exp $
--- custom/Makefile.in.orig	Wed Feb 11 17:03:52 2009
+++ custom/Makefile.in	Wed Feb 11 17:09:12 2009
@@ -11,7 +11,6 @@ Kernel=../boot/$m/kernel.o
 Custom=../boot/$m/custom.o
 PetiteBoot=../boot/$m/petite.boot
 SchemeBoot=../boot/$m/scheme.boot
-Archive=../zlib/$m/libz.a
 
 # force local rebuild of executable if desired
 ForceRelink=no