Makefiles: Don't complain if depend file to be included doesn't exist.

Makefiles: Don't complain if depend file to be included doesn't exist.

While bootstrapping the dependencies, make will see the "include depend"
directive before the depend file has been created. To avoid a spurious
warning in this case we use "-include" instead, (which differs precisely
in the fact that it will not emit a diagnostic if the named file does
not exist).