tags: new var $(am__tagged_files) to list all files to run taggers on

tags: new var $(am__tagged_files) to list all files to run taggers on

This change reduce code duplication a little, and will be needed by
future simplifications.

* automake.in (handle_tags): Define a new 'am__tagged_files' private
make variable that lists all files to run taggers (mkid, cscope, ctags,
etc) on.  Drop the now-useless transform '%CONFIG%' when including
'tags.am'.
* lib/am/tags.am: Use it in several recipes and dependencies list to
reduce code duplication.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>