[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.bor b/scripts/makefile.bor
index c4eed60..a01f738 100644
--- a/scripts/makefile.bor
+++ b/scripts/makefile.bor
@@ -117,7 +117,7 @@
 
 # try !include scripts\pnglibconf.mak for more options
 pnglibconf.h: $(PNGLIBCONF_H_PREBUILT)
-	$(CP) $< $@
+	$(CP) $(PNGLIBCONF_H_PREBUILT) $@
 
 libpng: $(LIBNAME)