maint: use more perl subroutines prototypes in the automake script

maint: use more perl subroutines prototypes in the automake script

* automake.in: Throughout this file.  Note that these new prototypes
are not much useful, since many subroutine calls still use the old
'&foo' form; but we'll take care of that in later patches.
* lib/Automake/Language.pm (target_hook): Call the '_target_hook'
of the given language in a more modern form, avoiding '&'.

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