since CMake 2.8.3 (according to Raphael) FindLibArchive module is
bundled with it, to avoid dublication drop the bundled one.
in addition, libarchive does not conditionally define support of
archives and will just bail out if the format is not recognized
thus dropping the ifdef's in the handler
TODO: the format does not get recognized via archive_format() when
called on existing archive created with Ark, what the heck? is it
a libarchive bug (test with git checkout)? if it is then checking
the format in the case when the filter is ARCHIVE_FILTER_NONE with
another switch would do the job without manually checking the
extensions for zip and 7z.
