[libpng16] $(CP) is now used alongside $(RM_F).  Also, use 'copy' instead

of 'cp' where applicable, and applied other minor makefile changes.
diff --git a/scripts/makefile.openbsd b/scripts/makefile.openbsd
index c0eb5ab..5599d75 100644
--- a/scripts/makefile.openbsd
+++ b/scripts/makefile.openbsd
@@ -36,7 +36,7 @@
 
 # see scripts/pnglibconf.mak for more options
 pnglibconf.h: scripts/pnglibconf.h.prebuilt
-	cp scripts/pnglibconf.h.prebuilt $@
+	cp $< $@
 
 pngtest.o:	pngtest.c
 	${CC} ${CPPFLAGS} ${CFLAGS} -c ${.ALLSRC} -o ${.TARGET}