DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)
diff --git a/scripts/makefile.hp64 b/scripts/makefile.hp64
index 6c15a0b..5948be8 100644
--- a/scripts/makefile.hp64
+++ b/scripts/makefile.hp64
@@ -34,6 +34,7 @@
 MKDIR_P=mkdir -p
 LN_SF=ln -sf
 RANLIB=ranlib
+CP=cp
 RM_F=/bin/rm -f
 
 CPPFLAGS=-I$(ZLIBINC) \
@@ -67,6 +68,10 @@
 DL=$(DESTDIR)$(LIBPATH)
 DM=$(DESTDIR)$(MANPATH)
 
+# Pre-built configuration
+# See scripts/pnglibconf.mak for more options
+PNGLIBCONF_H_PREBUILT = scripts/pnglibconf.h.prebuilt
+
 OBJS = png.o pngset.o pngget.o pngrutil.o pngtrans.o pngwutil.o \
 	pngread.o pngrio.o pngwio.o pngwrite.o pngrtran.o \
 	pngwtran.o pngmem.o pngerror.o pngpread.o
@@ -83,9 +88,8 @@
 
 all: libpng.a $(LIBSO) pngtest libpng.pc libpng-config
 
-# 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) $@
 
 libpng.a: $(OBJS)
 	$(AR_RC) $@ $(OBJS)
@@ -102,7 +106,7 @@
 	( cat scripts/libpng-config-head.in; \
 	echo prefix=\"$(prefix)\"; \
 	echo I_opts=\"-I$(INCPATH)/$(LIBNAME)\"; \
-	echo ccopts=\"-Ae +DA1.1 +DS2.0\"; \
+	echo ccopts=\"-O -Ae -Wl,+vnocompatwarnings +DD64 +Z\"; \
 	echo L_opts=\"-L$(LIBPATH)\"; \
 	echo libs=\"-lpng16 -lz -lm\"; \
 	cat scripts/libpng-config-body.in ) > libpng-config