top-level temporary directory for creating built distributions. (Won't
work yet, since the "build" command doesn't yet have a 'build_bdist'
option, and none of the "bdist" commands support it yet.)
* improve help strings
* warn if user supplies non-existing directories
* don't try to 'remove_tree()' non-existing directories
* try to remove the build_base after cleanup (but don't do or say
anything if it fails -- this is just in case we made it empty)
Greg Ward
Gregory P. Smith