[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.beos b/scripts/makefile.beos
index edac365..01346f1 100644
--- a/scripts/makefile.beos
+++ b/scripts/makefile.beos
@@ -87,7 +87,7 @@
 all: libpng.a $(LIBSO) pngtest libpng.pc libpng-config
 
 pnglibconf.h: $(PNGLIBCONF_H_PREBUILT)
-	$(CP) $< $@
+	$(CP) $(PNGLIBCONF_H_PREBUILT) $@
 
 libpng.a: $(OBJS)
 	$(AR_RC) $@ $(OBJS)