[Windows] Don't use a .def file

lib/Makefile

@@ -151,8 +151,7 @@ ifneq (,$(filter Windows%,$(OS)))
 LIBZSTD = dll\libzstd.dll
 $(LIBZSTD): $(ZSTD_FILES)
 	@echo compiling dynamic library $(LIBVER)
-	@$(CC) $(FLAGS) -DZSTD_DLL_EXPORT=1 -shared $^ -o $@
-	dlltool -D $@ -d dll\libzstd.def -l dll\libzstd.lib
+	$(CC) $(FLAGS) -DZSTD_DLL_EXPORT=1 -Wl,--out-implib,dll\libzstd.lib -shared $^ -o $@
 
 else