vim-patch:8.1.0539: cannot build without the sandbox

Problem:    Cannot build without the sandbox.
Solution:   Set the secure option instead of using the sandbox.  Also restrict
            the characters from 'spelllang' that are used for LANG.vim.
            (suggested by Yasuhiro Matsumoto)
82e8c92ebe

runtime/doc/options.txt

@@ -5561,7 +5561,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 
 	After this option has been set successfully, Vim will source the files
 	"spell/LANG.vim" in 'runtimepath'.  "LANG" is the value of 'spelllang'
-	up to the first comma, dot or underscore.
+	up to the first character that is not an ASCII letter and not a dash.
 	Also see |set-spc-auto|.
 
 

src/nvim/buffer.c

@@ -4945,11 +4945,11 @@ chk_modeline (
       save_SID = current_SID;
       current_SID = SID_MODELINE;
       // Make sure no risky things are executed as a side effect.
-      sandbox++;
+      ++secure;
 
       retval = do_set(s, OPT_MODELINE | OPT_LOCAL | flags);
 
-      sandbox--;
+      --secure;
       current_SID = save_SID;
       if (retval == FAIL)                     /* stop if error found */
         break;

src/nvim/option.c

@@ -3296,13 +3296,15 @@ ambw_end:
        * '.encoding'.
        */
       for (p = q; *p != NUL; ++p)
-        if (vim_strchr((char_u *)"_.,", *p) != NULL)
+        if (!ASCII_ISALPHA(*p) && *p != '-')
           break;
+      if (p > q) {
         vim_snprintf((char *)fname, sizeof(fname), "spell/%.*s.vim",
                      (int)(p - q), q);
         source_runtime(fname, DIP_ALL);
       }
     }
+  }
 
   if (varp == &p_mouse) {
     if (*p_mouse == NUL) {