When installing COPYING, recommend adding the file to VCS.
* automake.in (require_file_internal): If installing `COPYING',
mention that we install the GPLv3 file and recommend adding the
file to version control.
* doc/automake.texi (Invoking Automake): Point to `Gnits' node
for `--add-missing'.
(Gnits): Clarify semantics: that for strictness gnu or higher,
INSTALL is installed, and that COPYING is installed as GPLv3
if no COPYING file exists.
* tests/license2.test: New test.
* tests/Makefile.am: Update.
* NEWS, THANKS: Update.
Report by Brian Cameron.
Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>