Doesn't build without -std=gnu99 on gcc-4.7, and fails at many other
places when gnu99 is used. Better patch this one.

--- ./src/main.c.orig	2020-04-13 16:31:54.665877209 +0200
+++ ./src/main.c	2020-04-13 16:32:26.837853986 +0200
@@ -167,6 +167,8 @@
 
 static void show_help(const char *name)
 {
+	size_t i;
+
 	printf("Usage: %s [ options ] [ cmds... ]\n"
 	       "\n"
 	       "Options:\n", name);
@@ -183,7 +185,7 @@
 
 	fputs("\n", stdout);
 
-	for (size_t i = IDX_INTERACTIVE + 1; i < NR_NFT_OPTIONS; ++i)
+	for (i = IDX_INTERACTIVE + 1; i < NR_NFT_OPTIONS; ++i)
 		print_option(&nft_options[i]);
 
 	fputs("\n", stdout);