patch 8.1.1578: MS-Windows: pathdef.c should depend on build options

Problem:    MS-Windows: pathdef.c should depend on build options.
Solution:   Generate pathdef.c in the object directory.  Fix dependencies.
            (Ken Takata, closes #4565)
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 54740d7..04e566b 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -1702,7 +1702,7 @@
 E_LINKARGS2 = $(E0_LINKARGS2:"=\")
 # ") stop the string
 
-$(PATHDEF_SRC): auto
+$(PATHDEF_SRC): Make_mvc.mak
 	@echo creating $(PATHDEF_SRC)
 	@echo /* pathdef.c */ > $(PATHDEF_SRC)
 	@echo #include "vim.h" >> $(PATHDEF_SRC)
@@ -1713,9 +1713,6 @@
 	@echo char_u *compiled_user = (char_u *)"$(USERNAME)"; >> $(PATHDEF_SRC)
 	@echo char_u *compiled_sys = (char_u *)"$(USERDOMAIN)"; >> $(PATHDEF_SRC)
 
-auto:
-	if not exist auto/nul mkdir auto
-
 # End Custom Build
 proto.h: \
 	proto/arabic.pro \