maint: more files excluded in 'update-copyright'

maint: more files excluded in 'update-copyright'

This will avoid the following spurious warnings when
"make update-copyright" is run:

    .autom4te.cfg: warning: copyright statement not found
    .git-log-fix: warning: copyright statement not found
    .gitattributes: warning: copyright statement not found
    .gitignore: warning: copyright statement not found
    AUTHORS: warning: copyright statement not found
    THANKS: warning: copyright statement not found
    contrib/README: warning: copyright statement not found
    contrib/multilib/README: warning: copyright statement not found
    doc/amhello/README: warning: copyright statement not found
    lib/install-sh: warning: copyright statement not found
    lib/mkinstalldirs: warning: copyright statement not found
    m4/acdir/README: warning: copyright statement not found
    t/README: warning: copyright statement not found

* Makefile.am (files_without_copyright): New.
(update-copyright): Use it when computing '$exclude_list'.
Also, skip any README file.

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