[ng] automake: new global variable '%known_ltlibraries'

[ng] automake: new global variable '%known_ltlibraries'

This is just a preparatory refactoring in view of future changes.

* automake.in (%known_ltlibraries): New global variable.
(initialize_per_input): Reset it.
(handle_ltlibraries): Register processed libtool libraries in
'%known_ltlibraries' rather than in '%known_libraries'.
(lang_vala_finish): Also take into account the contents of
'%known_ltlibraries'; since we are at it, refactor a little
to remove some inconsistency and code duplication.

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