configure: fix defaults in help.

Both "[ 1 ]" and "[ 0 ]" both evaluate true in shell; we really want
an explicit '1' test.

Before:
$ ./configure --help
...
  --enable/disable-compat (default disable)

After:
$ ./configure --help
...
  --enable/disable-compat (default enable)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

configure

@@ -19,10 +19,10 @@ CONFIG_HEADER=ccan/config.h
 usage_with_default()
 {
     if [ $# = 4 ]; then
-	if [ $2 ]; then
-	    DEF=$4
-	else
+	if [ "$2" = 1 ]; then
 	    DEF=$3
+	else
+	    DEF=$4
 	fi
     else
 	DEF=$2