avoid MSVC build error

```
Microsoft (R) Incremental Linker Version 14.00.24241.7
Copyright (C) Microsoft Corporation.  All rights reserved.
io.def : error LNK2001: unresolved external symbol luaopen_compat53_io
compat53/io.lib : fatal error LNK1120: 1 unresolved externals
Error: Build error: Failed compiling module compat53/io.dll
```

lprefix.h

@@ -195,6 +195,9 @@ LUAMOD_API int luaopen_compat53_string (lua_State *L) {
 #  endif
 
 /* choose which popen implementation to pick */
+#  if defined(_MSC_VER)
+#    define LUA_USE_WINDOWS 1
+#  endif
 #  if (!defined(LUA_USE_WINDOWS) && !defined(LUA_USE_POSIX)) && \
       ((defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 200112L) || \
        (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE >= 600) || \
@@ -242,21 +245,24 @@ static void createmeta (lua_State *L);
 #  undef LUA_FILEHANDLE
 #  define LUA_FILEHANDLE COMPAT53_LUA_PFILEHANDLE
 
+LUAMOD_API int luaopen_compat53_io (lua_State *L) {
+  luaL_Reg const funcs[] = {
+
 /* for PUC-Rio Lua 5.1 only */
 #  if defined(LUA_VERSION_NUM) && LUA_VERSION_NUM == 501 && !defined(LUA_JITLIBNAME)
 
-LUAMOD_API int luaopen_compat53_io (lua_State *L) {
-  luaL_Reg const funcs[] = {
     { "popen", io_popen },
     { "type", io_ptype },
+
+#  endif /* for PUC-Rio Lua 5.1 only */
+
     { NULL, NULL }
   };
   luaL_newlib(L, funcs);
   createmeta(L);
   return 1;
 }
 
-#  endif /* for PUC-Rio Lua 5.1 only */
 
 /* fake CLANG feature detection on other compilers */
 #  ifndef __has_attribute