[ng] tags: move yet more processing to GNU make

[ng] tags: move yet more processing to GNU make

This will also simplify future changes aimed at making the current
'tags.am' makefile fragment included more "verbatim".

* lib/am/tags.am (am.tags.include-option): New internal variable, the
include-like option used by the $(EATGS) problem.  This must be
determined dynamically because Exuberant Ctags wants '--etags-include'
while GNU Etags wants '--include'.
(am.tags.subfiles): New, TAGS files in $(SUBDIRS) entries (if any)
that must be included in the top-level TAGS file.
(tags-am): Use them, and simplify accordingly.

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