depend: reduce code duplication, de-uglify make verbose output

depend: reduce code duplication, de-uglify make verbose output

Verbose output from compilation recipes had been made more messy by
the previous "simplifying" commit v1.13.1d-220-g1fa0c24.  With this
patch, not only we reduce some code duplication, but also improve
that output again, for the joy of users who dislike silent-rules :-)

* lib/am/depend2.am: Use the new '%XSOURCE%' transform, instead of
copying and pasting the idiom for VPATH rewrite emulation over and
over.  Fix some indentation and line-wrapping issues while at it.
* bin/automake.in (handle_languages): Implement the new transform.
* t/silent-nested-vars.sh: Adjust to avoid spurious failures.

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