configure: exit with error if OS is unknown

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@16807 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -1563,7 +1563,7 @@ case $target_os in
         ;;
 
     *)
-        target_os="${target_os}-UNKNOWN"
+        die "Unknown OS '$target_os'."
         ;;
 esac