zscanner: fix incompatible Makefile copy rule on BSD

src/zscanner/Makefile.am

@@ -18,10 +18,10 @@ CLEANFILES = descriptor.h descriptor.c
 nodist_libzscanner_la_SOURCES = descriptor.h descriptor.c
 
 descriptor.h: ../common/descriptor.h
-	cp $< $@
+	cp ../common/descriptor.h $@
 
 descriptor.c: ../common/descriptor.c
-	cp $< $@
+	cp ../common/descriptor.c $@
 
 if HAVE_RAGEL
 BUILT_SOURCES += scanner.c