Add quotes for -fconstexpr-depth= in an error message.

From-SVN: r264233

gcc/cp/ChangeLog

+2018-08-28  Martin Liska  <mliska@suse.cz>
+
+	* constexpr.c (cxx_eval_call_expression): Add quotes
+	to -fconstexpr-depth=.
+
 2018-09-10  Marek Polacek  <polacek@redhat.com>
 
 	* class.c (build_vtbl_ref): Remove.

gcc/cp/constexpr.c

@@ -1669,7 +1669,7 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
     {
       if (!ctx->quiet)
 	error ("%<constexpr%> evaluation depth exceeds maximum of %d (use "
-	       "-fconstexpr-depth= to increase the maximum)",
+	       "%<-fconstexpr-depth=%> to increase the maximum)",
 	       max_constexpr_depth);
       *non_constant_p = true;
       result = error_mark_node;