Unbreak with lua 5.2.

--- lpack.c.orig	Mon Jul 31 18:19:59 2006
+++ lpack.c	Tue May 17 09:18:25 2016
@@ -100,7 +100,11 @@ static int l_unpack(lua_State *L) 		/** unpack(s,f,[in
  const char *s=luaL_checkstring(L,1);
  const char *f=luaL_checkstring(L,2);
  int i=luaL_optnumber(L,3,1)-1;
+#if LUA_VERSION_NUM <= 501
  size_t len=lua_strlen(L,1);
+#else
+ size_t len=lua_rawlen(L,1);
+#endif
  int n=0;
  int swap=0;
  lua_pushnil(L);
@@ -251,7 +255,7 @@ static int l_pack(lua_State *L) 		/** pack(f,...) */
  return 1;
 }
 
-static const luaL_reg R[] =
+static const luaL_Reg R[] =
 {
 	{"pack",	l_pack},
 	{"unpack",	l_unpack},