patch 8.2.0245: MSVC: error message if the auto directory already exists

Problem:    MSVC: error message if the auto directory already exists.
Solution:   Add "if not exists". (Ken Takata, closes #5620)
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 3e8eae1..c51f804 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -1584,7 +1584,7 @@
 	$(CC) $(CFLAGS_OUTDIR) $(LUA_INC) if_lua.c
 
 auto/if_perl.c : if_perl.xs typemap
-	-mkdir auto
+	-if not exist auto/nul mkdir auto
 	$(XSUBPP) -prototypes -typemap $(XSUBPP_TYPEMAP) \
 		-typemap typemap if_perl.xs -output $@