That's why I've said "not just distribution guys".

I believe you just described how the Android build system works, except that they use a Python script to locate Android.mk files, rather than the find command. If you could find a way to apply this trick to autoconf projects, you probably could speed up quite a few builds-- at the cost of making things even more complex. The other alternative is just to use CMake, which will do this all for you, without any boilerplate. But I guess you're probably sick of hearing about that by now :)