[ng] distcheck: include it as a verbatim makefile fragment

[ng] distcheck: include it as a verbatim makefile fragment

* lib/am/distcheck.am: Rename ...
* lib/am/distcheck.mk: ... like this, and adjust to use GNU
make conditionals rather than Automake ones, and to allow for
user overrides of the '$(distcleancheck_listfiles)' variable.
Fix some comments while at it.
* automake.in (handle_dist): Include it suing the 'verbatim'
function.
* Makefile.am (dist_am_DATA): Adjust.
* t/distcheck-configure-flags-am.sh: Remove obsolete grepping
checks, that would cause the test to fails spuriously.
* t/distcheck-configure-flags.sh: Likewise.

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