[libpng16] Eliminated use of "$<" when copying $PNGLIBCONF_H_PREBUILT in

makefiles. This does not work on some versions of make; bug introduced in
libpng version 1.6.11.
diff --git a/scripts/makefile.gcc b/scripts/makefile.gcc
index 8811a28..ba92d15 100644
--- a/scripts/makefile.gcc
+++ b/scripts/makefile.gcc
@@ -43,7 +43,7 @@
 all: static
 
 pnglibconf.h: $(PNGLIBCONF_H_PREBUILT)
-	$(CP) $< $@
+	$(CP) $(PNGLIBCONF_H_PREBUILT) $@
 
 .c.o:
 	$(CC) -c $(CPPFLAGS) $(CFLAGS) -o $@ $<