configure: exit with error if OS is unknown

Originally committed as revision 16807 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

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