alien.libraries.finder: automatically find library files based on library names

To make it easier to support cross-platform library bindings use the
find-library word. Factor is then supposed to automatically locate the
correct library file depending on the users' platform. Then you dont
have to specify the full so-name on unixes which breaks if the user
has a newer library version. The same library lookups that
ctypes.find_library uses will be implemented in future commits.
Merge branch 'library-resolving' of https://github.com/bjourne/factor