Make guessing the target an info instead of a warning

Having this as a warning makes IDEs to go to the Makefile where the
warning was issued as if an error occurred, which is rather annoying for
something is really normal. An info is enough to let the user now how
the target was guessed.
Merge pull request #1797 from leandro-lucarella-sociomantic/cpu-warn-to-info

[trivial] Make guessing the target an info instead of a warning