[ng] cleanup: after enabling of subdir-objects unconditionally

[ng] cleanup: after enabling of subdir-objects unconditionally

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

* automake.in (LANG_IGNORE, LANG_SUBDIR): Remove.
(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): Don't expect the 'lang_*_rewrite' subroutines
to return a 'LANG_*' constant anymore, 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 relying on ...
(lang_vala_ignore, lang_header_ignore): ... these new subroutines to avoid
extra processing of C/C++ headers and Vala '.vapi' headers.
(lang_c_rewrite): Adjust to explicitly return an undefined value.
(lang_java_rewrite): Remove.
Remove an outdated comment.

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