DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)
diff --git a/contrib/arm-neon/linux.c b/contrib/arm-neon/linux.c
index 182f376..94f9bb1 100644
--- a/contrib/arm-neon/linux.c
+++ b/contrib/arm-neon/linux.c
@@ -2,7 +2,7 @@
  *
  * Copyright (c) 2014 Glenn Randers-Pehrson
  * Written by John Bowler, 2014.
- * Last changed in libpng 1.6.10 [March 6, 2014]
+ * Last changed in libpng 1.6.16 [December 22, 2014]
  *
  * This code is released under the libpng license.
  * For conditions of distribution and use, see the disclaimer
@@ -16,8 +16,8 @@
  * png_have_neon implemented for Linux by reading the widely available
  * pseudo-file /proc/cpuinfo.
  *
- * This code is strict ANSI-C and is probably moderately portable, it does
- * however use <stdio.h> and assumes that /proc/cpuinfo is never localized.
+ * This code is strict ANSI-C and is probably moderately portable; it does
+ * however use <stdio.h> and it assumes that /proc/cpuinfo is never localized.
  */
 #include <stdio.h>
 
@@ -72,7 +72,7 @@
                      state = Colon;
                   break;
                }
-               
+
                /* did not match 'feature' */
                state = SkipLine;
                /* FALL THROUGH */
@@ -152,8 +152,10 @@
       }
    }
 
+#ifdef PNG_WARNINGS_SUPPORTED
    else
       png_warning(png_ptr, "/proc/cpuinfo open failed");
+#endif
 
    return 0;
 }