let CFLAGS autodetection choke on stack protector when the libc lacks it

(Linux klibc) by introducing an array on the stack of main()

Build.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
-srcversion='$MirOS: src/bin/pax/Build.sh,v 1.23 2021/10/03 20:48:05 tg Exp $'
+srcversion='$MirOS: src/bin/pax/Build.sh,v 1.24 2021/10/11 22:23:05 tg Exp $'
 #-
 # Copyright (c) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
 #		2011, 2012, 2013, 2014, 2015, 2016, 2017, 2019,
@@ -339,7 +339,7 @@ ac_flags() {
 		ac_testn can_$vn '' "$ft" <<-'EOF'
 			/* evil apo'stroph in comment test */
 			#include <unistd.h>
-			int main(void) { return (isatty(0)); }
+			int main(void) { int t[2]; return (isatty(pipe(t))); }
 		EOF
 		#'
 	fi
@@ -399,6 +399,7 @@ addsrcs() {
 	esac
 }
 
+# --- main ---
 
 curdir=`pwd` srcdir=`dirname "$0" 2>/dev/null`
 case x$srcdir in