[ng] push_dist_common: don't directly append to an Automake::Variable

[ng] push_dist_common: don't directly append to an Automake::Variable

This change should cause no semantic changes; it just makes few lines of
code a little cleaner and clearer.

* automake.in ($configure_dist_common): Turn this global scalar into ...
(@configure_dist_common): ... a global array.
(@dist_common): New global, holding the list of the "auto-discovered"
distributed files.
(initialize_per_input): Clear it.
(scan_autoconf_files): Use it, instead of the value of the make variable
'am__dist_common', to initialize '@configure_dist_common'.
(handle_dist): New %transform 'DIST-COMMON => "@dist_common"' for the
'distdir.am' file.
(push_dist_common): Do not call 'Automake::Variable::define' (with "+"
type) on 'am__dist_common' anymore; just append to @dist_common instead.
Adjust few comments throughout the file.
* lib/am/distdir.am: Now that automake.in doesn't define 'am__dist_common'
anymore, initialize it with the contents of the '%DIST-COMMON%' transform.

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