cleanup: having subdir-objects mandatory allow us some simplifications

cleanup: having subdir-objects mandatory allow us some simplifications

Few minor cleanups made possible by earlier changes, plus other minor
cleanups triggered in cascade.  No semantic change is intended.

This is a follow-up on previous commit 'v1.13.1d-214-g3ad07d2', and
an adjusted backport of Automake-NG commit 'v1.12.1-315-gc97d41b'
of 2012-06-08 ([ng] cleanup: after enabling of subdir-objects
unconditionally).

* bin/automake.in (LANG_IGNORE, LANG_SUBDIR): Remove.
(handle_languages): Drop the '%DEPBASE%' transform when processing
the '$rule_file'.
(register_language ('name' => 'vala', ...)): Add '.vapi' to the entry
'extensions', and simplify the entry 'output_extensions' to point to
a dummy subroutine (since it wasn't really used anyway).
(handle_single_transform): No longer expect the 'lang_*_rewrite'
subroutines to return a 'LANG_*' constant, but only a transformed
extension, if required.  To decide whether further processing of the
source file should be stopped, rely on a new set of 'lang_*_ignore'
subroutines, defaulting to a subroutine that returns false.
Accordingly, don't special case the handling of '.vapi' files anymore,
instead rely on ...
(lang_vala_ignore, lang_header_ignore): ... these new subroutines to
avoid extra processing of C/C++ headers and Vala '.vapi' headers.
(lang_java_rewrite): Remove.
Remove an outdated comment.
* lib/am/depend2.am: Partial rewrite to reduce code duplication and
drop use of the '%DEPBASE%' transform.
* t/compile_f_c_cxx.sh: Adjust.

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