--- configure.orig	2009-10-30 11:51:40.000000000 -0700
+++ configure	2013-09-12 09:08:52.000000000 -0700
@@ -5109,6 +5109,7 @@
 
 { echo "$as_me:$LINENO: checking for zlib.h" >&5
 echo $ECHO_N "checking for zlib.h... $ECHO_C" >&6; }
+unset ac_cv_header_zlib_h
 if test "${ac_cv_header_zlib_h+set}" = set; then
   echo $ECHO_N "(cached) $ECHO_C" >&6
 else