configury: don't nuke the lib subdirectory.

Now that we've made use of lib for Lua library files, make sure
not to let gnulib-tool scribble in it.
* bootstrap.conf (source_base): Tell gnulib-tool to put its empty
library directory somewhere harmless.
(luaposix_remove_empty_lib): And then remove that.

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