For PR automake/357:

For PR automake/357:
* automake.in (require_file_internal): Don't complain a file is "not
found" when it is a target.  Make sure "not found" files are otherwise
reported as error (they were reported as 'note').
* tests/distcommon3.test: Augment to test the above.
* tests/libobj12b.test: Touch ltmain.sh.
* tests/required2.test: Make sure Automake fails because ltmain.sh
has been removed.
Reported by Ronald Landheer.