Guile 2.x support thin-edge

The infrastructure for supporting different Guile versions seems solid
for Guile 1.4 and 1.8 (and maybe 1.6, but no one has reported on that to
date).  Now, let's see how it handles 2.0:

http://git.savannah.gnu.org/cgit/serveez.git/commit/?h=next&id=5daee6b