FPII-2034: DO NOT MERGE Update libpng to 1.6.20

DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I2fc2106965c789a54f04412e9c63a0901b0c1e71
(cherry picked from commit 0b39f202aaedd3029e4270d2af7d58ba6634f9cd)
diff --git a/contrib/examples/pngtopng.c b/contrib/examples/pngtopng.c
index b1b3be6..4acf6b3 100644
--- a/contrib/examples/pngtopng.c
+++ b/contrib/examples/pngtopng.c
@@ -20,6 +20,8 @@
  * ensure the code picks up the local libpng implementation:
  */
 #include "../../png.h"
+#if defined(PNG_SIMPLIFIED_READ_SUPPORTED) && \
+    defined(PNG_SIMPLIFIED_WRITE_SUPPORTED)
 
 int main(int argc, const char **argv)
 {
@@ -90,3 +92,4 @@
 
    return result;
 }
+#endif /* READ && WRITE */