depend: refactor and remove code duplication

depend: refactor and remove code duplication

* lib/am/depend2.am: Here.  The obsolescent comments removed from this
same file in commit 'v1.13.1-42-g8f06bfb' of 2012-01-09, "depend2.am: fix
comments on verbosity of compilation rules", gave the rationale for why
that code duplication was there in the first place (rationale that, like
those comments, has been obsolete by the silent-rules introduction).
* bin/automake.in: Given the refactoring in 'depend2.am', there is no
longer need to add extra trailing whitespace to the entries of the
'%sourceflags' hash (which are used for the '%SOURCEFLAG%' transform
when 'depend2.am' is processed.
* t/fort2.sh: Adjust to avoid spurious failures.

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