Patch from Andrew Kuchling: prune out the build and source distribution

Patch from Andrew Kuchling: prune out the build and source distribution
directories after all is said and done, so we don't accidentally include
those files in the source distribution.

(This is the quick and easy way to fix this; Andrew says: "Changing
findall() looked like it was going to be messy, so I tried this instead.
The only problem is that redundant directory traversals are being done,
walking through build/ only to throw out all the files found at the end.").