configure: allow 'none' as target OS

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@21732 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -2217,7 +2217,8 @@ case $target_os in
         ;;
     gnu)
         ;;
-
+    none)
+        ;;
     *)
         die "Unknown OS '$target_os'."
         ;;