Use luaL_traceback only with Lua 5.2+

It has been added in Lua 5.2, and should not require us to bump the
Lua dependency.  It is only meant to provide useful information, but is
not required.

This is a follow-up to 6e97b5f8.

luaa.h

@@ -57,8 +57,10 @@ luaA_warn(lua_State *L, const char *fmt, ...)
     va_end(ap);
     fprintf(stderr, "\n");
 
+#if LUA_VERSION_NUM >= 502
     luaL_traceback(L, L, NULL, 2);
     fprintf(stderr, "%s\n", lua_tostring(L, -1));
+#endif
 }
 
 static inline int
@@ -66,8 +68,10 @@ luaA_typerror(lua_State *L, int narg, const char *tname)
 {
     const char *msg = lua_pushfstring(L, "%s expected, got %s",
                                       tname, luaL_typename(L, narg));
+#if LUA_VERSION_NUM >= 502
     luaL_traceback(L, L, NULL, 2);
     lua_concat(L, 2);
+#endif
     return luaL_argerror(L, narg, msg);
 }