Build project from top-level CMakeLists.txt

Created by: aszlig

This should (hopefully, haven't tested) be backwards-compatible (especially for the buildbot-scripts) with the current top-level Makefile and it introduces just a new CMakeLists.txt at the top-level plus adds a few more fixes that might come up in the future once distros are picking up libinsane.

The idea is to switch to the top-level CMakeLists.txt for the buildbot scripts and then unify everything into one CMake project, so that there is only the target for the actual library and glib. This should also get rid of a few hacks that have been made to for example reference libinsane from libinsane-gobject.

In the long term I guess it would also make sense to flatten the tree a bit, so there is only src for the actual library and glib for the GLIB bindings.