fix check for libpng

package/firefox/patches/patch-configure

@@ -1,5 +1,5 @@
---- mozilla-1.9.2.orig/configure	2010-12-03 16:57:11.000000000 +0100
-+++ mozilla-1.9.2/configure	2010-12-29 21:52:34.000000000 +0100
+--- mozilla-1.9.2.orig/configure	2011-03-19 21:39:59.000000000 +0100
++++ mozilla-1.9.2/configure	2011-04-23 11:35:36.000000000 +0200
 @@ -3970,7 +3970,7 @@ else
      echo "$ac_t""yes" 1>&6    
  fi
@@ -17,6 +17,17 @@
  HOST_AR_FLAGS='$(AR_FLAGS)'
  
  MOZ_JPEG_CFLAGS=
+@@ -13581,9 +13580,9 @@ if test "$SYSTEM_PNG" = 1; then
+     LIBS="$PNG_LIBS $LIBS"
+     cat > conftest.$ac_ext <<EOF
+ #line 13584 "configure"
+-#include "confdefs.h"
+  #include <stdio.h>
+                      #include <sys/types.h>
++#include "confdefs.h"
+                      #include <png.h> 
+ int main() {
+  #if PNG_LIBPNG_VER < $MOZPNG
 @@ -17895,7 +17894,7 @@ echo "configure:17889: checking for SQLI
  else