Replace ltdl with GLib's GModule

[Mike Gran]

Hello all-

In a fit of pique, I pushed a new branch of Guile to the repo
called wip-replace-ltdl-with-gmodule.  It replaces the dynamic linking
library libltdl from libtool with the analagous library GModule
from GLib.  It was remarkably easy, and, after a cursory test
I was surprised to find that it seems to be working.

GModule itself is a standalone library, but, I think it depends
on GLib.  It might be possible to remove that dependency without
much difficulty, but unfortunately, then you would be left with
another library to maintain.

Anyway, you may find it amusing, at least.

Regards,
Mike Gran