cli: display a warning whenever default output is stdout while input != stdin

cli: display a warning whenever default output is stdout while input != stdin

This behavior has been preserved for compatibility with existing ecosystem.
But it's problematic, as some environment start `lz4` without identifying stdout as console by default,
leading to a change of behavior for a same line of script.

A more sensible policy would be to default to stdout only when input is stdin.

Soft change for the time being : keep the behavior, just print a warning message.
User should prefer `-c` to explicitly select `stdout`.

Also : updated tests in Makefile to explicitly select `stdout` with `-c`.