Update to libpng 1.6.19

Removes all Android modifications, including
SK_PNG_INDEX_SUPPORTED (for subset decoding).

BUG:23265085
BUG:25424175
Change-Id: Iaa4f6cc9fc6db8e4b46a83211363adfcbf07b589
diff --git a/scripts/makefile.openbsd b/scripts/makefile.openbsd
index c61fa2e..d8824d2 100644
--- a/scripts/makefile.openbsd
+++ b/scripts/makefile.openbsd
@@ -11,7 +11,7 @@
 MANDIR= ${PREFIX}/man/cat
 
 SHLIB_MAJOR=	16
-SHLIB_MINOR=	1.6.10
+SHLIB_MINOR=	1.6.19
 
 LIB=	png
 SRCS=	png.c pngerror.c pngget.c pngmem.c pngpread.c \
@@ -31,12 +31,15 @@
 DOCS=	ANNOUNCE CHANGES INSTALL KNOWNBUG LICENSE README TODO Y2KINFO \
 	libpng-manual.txt
 
+# Pre-built configuration
+# See scripts/pnglibconf.mak for more options
+PNGLIBCONF_H_PREBUILT= scripts/pnglibconf.h.prebuilt
+
 .c.o:
 	$(CC) -c $(CPPFLAGS) $(CFLAGS) -o $@ $<
 
-# 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) $@
 
 pngtest.o:	pngtest.c
 	${CC} ${CPPFLAGS} ${CFLAGS} -c ${.ALLSRC} -o ${.TARGET}