* Rename prepare_sources.py to configure.py; the operation is similar to
autoconf ./configure so the association is useful.
* Rename make_dist.py to dist.py.
* Generate only one source set (combined or separate) for one run of
configure.py.
* Change dist.py to run configure.py three times to generate the default
source sets.
* Use an autodeleted temporary directory for preparing sources, which
removes some of the manual temporary file handling.