[ng] warns: don't report possible issues with '_DEPENDENCIES' variables

[ng] warns: don't report possible issues with '_DEPENDENCIES' variables

This will allow us to move (in future changes) some more processing at
make runtime, without having automake complain spuriously (which couldn't
be avoided, as to avoid such complaints automake should have access to
context and information only available at make runtime -- which is clearly
impossible).

* NG-NEWS: Document that Automake-NG can be weaker than mainline Automake
in diagnostic and early error detection; and also explain why it is so.
* automake.in (check_typos): Don't check for possible typos in the
'_DEPENDENCIES'.
Consequently, don't bother calling 'set_seen' ...
(handle_per_suffix_test): ... on the 'LOG_DEPENDENCIES' variables ...
(scan_aclocal_m4): ... nor on the 'CONFIG_STATUS_DEPENDENCIES' and
'CONFIGURE_DEPENDENCIES' variables ...
(handle_tags): ... nor on the 'TAGS_DEPENDENCIES' variable ...
(handle_ltlibraries): ... nor on the 'libfoo_la_DEPENDENCIES' nor
the 'EXTRA_libfoo_la_DEPENDENCIES' variables ...
(handle_libraries): ... nor on the 'libfoo_a_DEPENDENCIES' nor the
'EXTRA_libfoo_a_DEPENDENCIES' variables ...
(handle_programs): ... nor on the 'program_DEPENDENCIES' nor the
'EXTRA_program_DEPENDENCIES' variables.
* t/vartypos.sh, t/vartypo2.sh: Relax accordingly.

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