bootstrap: make sure gnulib file droppings are removed.

bootstrap: make sure gnulib file droppings are removed.

* bootstrap.conf (libtool_cleanup_empty_dirs): Recent bootstrap
updates set source_base to null, so we need to use ${x:-y} to
override the null.  Autoconf Shellology says that ancient BSD
/bin/sh chokes on :- defaults, but bootstrap is a developer tool,
and so we can reasonably expect a developer to have a working
/bin/sh to run the bootstrap script.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>