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)
diff --git a/src/buffer.c b/src/buffer.c
index ee962b2..8235a4f 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5523,11 +5523,11 @@
 		current_sctx.sc_lnum = 0;
 #endif
 		// Make sure no risky things are executed as a side effect.
-		++sandbox;
+		++secure;
 
 		retval = do_set(s, OPT_MODELINE | OPT_LOCAL | flags);
 
-		--sandbox;
+		--secure;
 #ifdef FEAT_EVAL
 		current_sctx = save_current_sctx;
 #endif