[ng] automake: remove useless code duplication

[ng] automake: remove useless code duplication

This is a pure refactoring with no semantic changes intended.

* automake.in (handle_all_and_check, handle_tags): Do not re-calculate
the list of "local" input or output header files (specified by
AC_CONFIG_HEADERS in configure.ac); that is already saved in the internal
make variables '$(am.config-hdr.local)' and '$(am.config-hdr.local.in)'.
Simply use these variables instead.

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