DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)
diff --git a/scripts/makefile.knr b/scripts/makefile.knr
index 7209215..d8de064 100644
--- a/scripts/makefile.knr
+++ b/scripts/makefile.knr
@@ -36,15 +36,21 @@
 RANLIB = ranlib
 #RANLIB = echo
 
+CP = cp
+RM_F = rm -f
+
+# 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
 
 all: ansi2knr libpng.a pngtest
 
-# 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) $@
 
 # general rule to allow ansi2knr to work
 .c.o:
@@ -83,7 +89,7 @@
 	chmod 644 $(DESTDIR)$(LIBPATH)/libpng.a
 
 clean:
-	rm -f *.o libpng.a pngtest pngout.png ansi2knr pnglibconf.h
+	$(RM_F) *.o libpng.a pngtest pngout.png ansi2knr pnglibconf.h
 
 DOCS = ANNOUNCE CHANGES INSTALL KNOWNBUG LICENSE README TODO Y2KINFO
 writelock: