[ng] tags: use more GNU make features

[ng] tags: use more GNU make features

* lib/am/tags.am (am__uniquify_input, am__define_uniq_tagged_files):
Remove, their role taken over by ...
(am.tags.files.uniq): ... this new (memoized) variable.
All rules adjusted.
(tags-am): Use $(CURDIR) instead of `pwd` to compute the absolute
path of the current working directory.
(GTAGS): Use $(abs_top_builddir) instead of `cd $(top_builddir) && pwd`
to compute the absolute path of the top-level build directory.

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