DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)
diff --git a/scripts/makefile.intel b/scripts/makefile.intel
index 1abfc6e..4187261 100644
--- a/scripts/makefile.intel
+++ b/scripts/makefile.intel
@@ -36,6 +36,11 @@
 CFLAGS=-O2 -G$(CPU)$(CALLING) -Qip -Qunroll4 -nologo
 LD=link
 LDFLAGS=/SUBSYSTEM:CONSOLE /NOLOGO
+CP=cp
+
+# Pre-built configuration
+# See scripts\pnglibconf.mak for more options
+PNGLIBCONF_H_PREBUILT = scripts\pnglibconf.h.prebuilt
 
 O=.obj
 
@@ -45,9 +50,8 @@
 
 all: test
 
-# see scripts/pnglibconf.mak for more options
-pnglibconf.h: scripts/pnglibconf.h.prebuilt
-	cp scripts/pnglibconf.h.prebuilt $@
+pnglibconf.h: $(PNGLIBCONF_H_PREBUILT)
+	$(CP) $(PNGLIBCONF_H_PREBUILT) $@
 
 png$(O): png.h pngconf.h pnglibconf.h pngpriv.h pngstruct.h pnginfo.h pngdebug.h
 	$(CC) $(CPPFLAGS) $(CFLAGS) $*.c $(ERRFILE)