Enter a warning about .deps files

It occured to me that a .deps file was made, even when the corresponding .c
file was not mentioned in any target. I suspect this to be because of the
-includes of .deps files which could trigger the .deps recipe for every .c
file in the directory.

If the .c file has wrong include paths, the .deps file will cause a DEPMAKE
of these missing includes and most probably, the make process will abort due
to errors.

This is something a developer must know and react upon. It also seems that
changing the .c file didn't help after the .deps file was created so it had
to be removed by hand (I didn't test this further).