[ng] refactor: remove all uses of '$(am__strip_dir)'

[ng] refactor: remove all uses of '$(am__strip_dir)'

Prefer using GNU make built-in '$(notdir)' instead.  This change doesn't
offer any serious simplification, but is just a step in the general
direction of moving more non-trivial processing to GNU make.

* lib/am/lib.am: Rewritten some install/uninstall rules to avoid
using '$(am__strip_dir)'.
* lib/am/lisp.am: Likewise.
* lib/am/ltlib.am: Likewise.
* lib/am/python.am: Likewise.
* lib/am/texinfos.am: Likewise.
* lib/am/header-vars.mk (am__strip_dir): Remove.

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