[ng] vars: simplify logic for appending conditionally

[ng] vars: simplify logic for appending conditionally

This will make the generated Makefiles more bloated, but that should
be OK, since eventually further refactorings will change the format used
to output conditional variables extending, and the resulting Makefiles
will once again be smaller and cleaner.

* lib/Automake/Variable.pm (define): When appending to an existing
variable *defined in another condition*, don't bother to avoid the
creation of intermediate internal variables when that's not really
needed; just create them unconditionally.
(_new): Don't reset $self->{'last-append'} anymore, it has been
removed (it was only used in the 'define' function).

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