cleanup: refactor code to initialize DIST_COMMON

cleanup: refactor code to initialize DIST_COMMON

There is not need to make that an Automake variable early,
only to later get and munge its contents, and use the new
content to redefine the variable.

* bin/automake.in (@dist_common): New global variable.
(push_dist_common, handle_dist): Use it.
(handle_dist): Define am__DIST_COMMON instead of DIST_COMMON
directly.
(initialize_per_input): Reset it to empty.
($configure_dist_common): Turn this scalar variable ...
(@configure_dist_common): ... into this array variable.
(handle_dist): Adjust.
(required_file_check_or_copy): Update and wrap some comments.
* lib/am/distdir.am (DIST_COMMON): Append $(am__DIST_COMMON).
* t/distcom2.sh: Tighten a little.

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